Although love triangles are the classic romantic conflict that others whisper about, those involved traditionally have maintained secrecy about their affairs.

But in the last few years, the situation has lost some of its clandestineness. Troubled triad members--inspired by the success of other mutual-help groups--are reaching out to those in similar circumstances.

In 1978, sociologist and ex-mistress Melissa Sands, 32, wrote The Mistress' Survival Manual and launched a national network called Mistresses Anonymous. Last year, Washington telecommunications consultant Sandra Michels, 23, co-founder of the now-defunct Organization of Other Persons (OOPs), conducted several Open University classes for all three members of a love triangle.

The most ambitious of the new "triangle support groups" is the Other Woman's Forum, started last fall in northern Virginia by film editor Susan Behr, 34, who ended her two-year affair in 1980, and administrator Pat Makin, 39, who walked away from her three-year affair in 1969.

"We didn't want to call ourselves mistresses," says Behr, "because it makes people think of mink and penthouse apartments, not real people who get swept up into this kind of thing. And we didn't want to use any term that implies a sickness because we consider an affair a life experience, not necessarily a problem."

The group's purpose, says Makin, is to "give women who are involved with ineligible men the opportunity to talk with others having the same experience so they can gain strength and insight. The idea is not to preach or tell people what to do. We want to help women focus on themselves and what they want, because usually the Other Woman's life revolves around the married man and his needs."

Their dream, says Behr, is that--in the tradition of Weight Watchers and Alcoholics Anonymous, whose organizing techniques they studied--"Someday we'll be running groups all around the country."

Although both women's affairs are over, each admits to continued "emotional fallout" from the experience. The two friends got the idea to start the forum because they had discussed their feelings about the subject frequently, and they were curious about how other women deal with it.

Like the affairs of many women who become involved with ineligible men, Behr's and Makin's started in a similar way: During a lonely, troubled time in their lives, a married man at the office offered affection, understanding and companionship with no strings attached. Despite some twinges of guilt, the prospect of having a secret soul mate--minus the confines of commitment--seemed exciting.

Each woman thought she could handle the situation, but after several months of intrigue--punctuated by romantic trysts and painful partings--each had fallen hopelessly in love.

"The worst part for me was the sense of powerlessness," recalls Behr. "My whole life revolved around his schedule."

"I always felt a nagging sense of guilt," says Makin. "When I ended it, I forced myself to shove the hurt and pain inside so I could go on with my life."

For both, a major problem was the inability to share their thoughts and feelings with others. "My family and my friends, including Pat, refused to discuss it," says Behr. "Affairs are so common, yet so clandestine and isolating. Even after my relationship ended, I found myself struggling with lots of unresolved issues. I had a strong desire to see how other women dealt with the situation."

In October they placed newspaper ads announcing the forum and received more than 100 responses. They hired a social worker to run the groups ("We're organizers," says Behr, "not qualified group leaders") and held the first meeting in January. Participants were required to sign a pledge of confidentiality, pay a $15 membership fee and $8 per one-hour meeting.

"But only a fraction of the people who called came," says Behr. "Many were afraid of being seen by someone they knew. One woman said she almost didn't come in because it meant admitting she was involved in this kind of situation. Also, we held the meetings on weekday nights, which is when 'other women' get to see their men."

After they moved meetings to weekends, attendance improved. Currently 45 members attend meetings held each weekend in rented rooms in the District and Virginia. There are plans to start a newsletter for women reluctant to appear in person.

The Other Women's Forum is one of an estimated half-million mutual-help groups in existence. Social scientists attribute their popularity to social change--such as divorce and high mobility--that have altered traditonal support patterns, and to the increased complexity and pressures of modern life.

"The abilty to adjust to a difficult situation or life change requires empathy from others far more than it does sympathy," says a report by the National Institute of Mental Health. "Stepping into the security of such a group can be like coming home for those who have been too long isolated by their private, painful concerns . . . from there, they can begin to communicate more openly, view their problems more objectively and find more effective coping strategies."

"Other women" have been relatively slow to reach out to one another, probably as a result of the secrecy surrounding most affairs. But "there are lots of hurting women out there dying to talk to someone," says C.C. Dyer, an associate producer of ABC-TV's "20/20" who is coordinating a segment on mistresses.

"After one ad in The Washington Post we got a couple hundred calls. Some of the women practically exploded and said I was the first person they've told in five, 10 years."

As Melissa Sands asserts in her Survival Manual: "Every mistress thinks that she can't tell anyone about her secret life. She can. She should. Because she is not alone."

Women involved in affairs often can't confide in the people with whom they usually talk, says Sands, "because they phase out friends who will not condone their relationship. Many women go to therapists, but not all can afford it. And psychiatrists insist on looking at women in Freudian terms. They'll tell her she's got a father fixation. Psychologists just keep saying, 'What's the problem?'"

Usually, says Sands, the one person a mistress can talk openly with is another mistress. "Mistresses use a sort of telepathy to find each other."

Locating another mistress is relatively easy, she claims, since "the number of mistresses is rising" as a result of the high divorce rate and the flood of women into the work force. "Divorce's who are emotionally hurt often choose relationships with married men because they think they're safe--until they fall in love.

"And today's offices are passion factories. Ninety percent of affairs start at work." (One national magazine calls the phenomenon "the hot bed of the '80s.")

Feminists, believes Sands, "may be more vulnerable. Self-supporting and self-sufficient . . . all she needs from a man is affection and warmth--or so she thinks at first."

Among other statistics, compiled by Sands after talking with "hundreds and hundreds of mistresses":

* About 40 percent are single and never married; 33 percent, divorced; 3 percent, widowed; 25 percent, married.

* About 16 percent are age 17 to 21; 37 percent in their twenties; 27 percent in their thirties; 13 percent in their forties; 6 percent in their fifties; 2 percent over 60.

* Only 2 percent are supported financially by their lover.

Most (65 percent) have only one affair with a married man.

* Most affairs last three to 11 years.

Sands corresponds with hundreds of mistresses from around the country through her Triangle Tabloid, the official newsletter of Mistresses Anonymous. Articles cover topics such as "Surviving the Holidays," "Guilt to the Hilt" and "Questionnaire for the Married Man."

Although Sands' mail, she says, includes requests from "women in every state in the union who want me to bring MA there," she gave up on running mistress' support groups several years ago after her initial effort "wound up attracting lots of reporters and suspicious wives. The need for privacy is so strong, it's a very difficult thing to pull off."

Unlike the Other Woman's Forum, which makes no judgments about whether to continue or end an affair, Sands' goal is to get women to "explode the triangle . . . either get the married man or get out."

Being a mistress "is like being a junkie," contends Sands, who wound up marrying the man with whom she had her affair. "It's a roller-coaster existence where you find you've developed a dependency that you just never bargained for.

"Mistressing is a phase that fits into a certain part of a woman's life. I got to a point where I needed to resolve my triangle, and I think most mistresses get to that point, too. I can't promise mistresses that they'll get their married man, but I can promise them peace and the chance to find happiness with a man who will be exclusively theirs."

A major reason people have affairs, maintains Sandra Michels, who last year ran Open University classes called "Triangles: Untangling the Web," is "an epidemic of commitment-phobia. In my case, I had several relationships where I got very hurt. So I decided affairs were the answer.

"My parents got divorced when I was 13, so that reinforced the message that commitment didn't work. By having an affair, I could have a man in my life without the hassle of expectations. We'd be good friends and have good sex. Period.

"But the problem for me, as a feminist, was feeling bad about hurting another woman. And despite all the intellectualizing that goes on, you still fall in love--or at least get emotionally attached--and you still get hurt."

Michels began sharing these thoughts with housemates of both sexes several years ago when she was an economics major at Brandeis University. "Nearly everyone I knew was dealing with this issue," she says, "or was close to someone who was. Sometimes affairs were an important growth experience. Other times they were an emotional crutch."

About a dozen of the students formed OOPs, which met regularly for about six months. "We weren't telling people to have--or not to have--affairs," she says. "We were trying to help people cope with a societal phenomenon and adapt to a complex world where there are no rules--so you have to make your own."

When she moved to Capitol Hill in 1981, Michels restarted the group by offering classes through Open University: one for men, one for women and one for both.

"The same-sex groups were great," she says, "but the mixed-sex group never really got going. The men would intellectualize, and the women would talk about how they felt. Neither would really talk to the other."

The best session, says Michels, was the women's. "About six women signed up, but only four came. As soon as I started talking, one well-dressed woman in her late thirties started to cry. She was a Catholic, her husband was having an affair with his secretary and she had no one to talk to. She couldn't go to the clergy or her friends. She felt like she'd lost her dignity, that she was a failure."

Michels encouraged a dialogue between this woman and a secretary in the group having an affair with her boss.

"I was a little scared they'd be at each other's throats. But it was great. They really listened to each other and got a lot out of their systems. The evening ended with everyone feeling good, like they'd learned something."

After talking with "dozens of people" involved in triangles, Michels says "a lot of them want to end it, but don't have the guts. Often people can handle it intellectually, but emotionally they're a mess. I tried to help people sort out how they felt, and use that as their guide for what to do."

OOPs folded after four months. "I didn't want it to be my claim to fame," says Michels. "And I felt I'd resolved a lot of issues for myself, which was one of the reasons I got into it.

"After my own experience, and talking about it with lots of other women and men, I'm a firm believer in monogamy."