Three years ago Heart was playing at Seattle's Aquarius, the largest tavern in Washington state. The Aquarius is at the center of the Northwest bar circuit and Heart seemed destined to play forever in similar clubs, covering other people's hits and trying to take out their earnings.

Today, the two-woman, four-man rock group could probably buy a dozen bars like Aquarius because nearly 5 million Heart records have been bought by their fans, 17,000 of whom are expected at the Capital Centre tonight for Heart's first concert here.

With Boston, Heart was the most successful new rock group to emerge in 1976. Boston arrived on the strength of a hit single, "More Than A Feeling," that assimilated enough musical touchstones to reach a diverse audience. Heart built slowly through a series of singles: "Magic Man," "Crazy on You," "Dreamboat Annie" and most recently "Barracuda."

The roots of Heart's success, along with the group's uncommon make-up constitute a musical version of "Rocky." Although certain members of the band have been playing together since 1963. Heart did not jell until the "70s and the arrival of the Wilson sisters. Ann, the older sister, lead vocalist and songwriter, joined in 1970, with guiter-player Nancy coming in four years later.

Backed by a lot of air play and constant touring - the group has averaged 200 dates a year for the past two years - "Dreamboat Annie," Heart's debut album, delivered three hit singles and has now sold over 2.5 million copies.

Part of the price has been a personal dilemma for members of the group, a result of the coverage Heart received in the pop press as it found itself in the national limelight.

Radio may have been responsible for record sales, but it was through advertising and features in music magazines that Heart's image was developed.

Things got off to a rocky start when Mushroom - the label that had signed the group in Canada - placed a series of national ads in a National Enquirer-type format with the Wilsons in a pose of sexual exhilaration and the caption "Dreamabout Annie' Goes Platinum: It Was Our First Time."

The photo spreads on the group tended to resemble "Mademoiselle" ads, and the features themselves emphasized the Wilson's feminity over their considerable talents. Both women are attractive and both have been presented as modern-day vamps for whom music is a secondary pleasure

"That's human nature," Ann Wilson says resignedly of the critics' blas. "I don't know whether they could be persuaded by anything we could ever say to do anything more. Hopefully, the music itself will entice them eventually."

There are parallels and precedents. Fleetwood Mac, with its daytime television serial and intra-band dramas, may be most responsible for this attention to "girl groups." Heart has been hailed as the first woman-dominated hard-rock band though they are in fact neither woman-dominated (Nancy is a rythm, not a lead guitarist) nor restricted to a hard-rock format. They are merely a manifestation of the recent plethora of immensely successful co-sexual banks, of which they and Fleetwood Mac are the most visible.

Looking back at women rockers like Suzie Quatro, April Lawton and the group Fanny, Ann Wilson feels that "they came off too super-sexy to be real and therefore could have only a short-term sort of appeal. These days, when people look at us or Fleetwood Mac or Abba, groups that feature women, they see something they can relate to more. It adds longevity and reverses that trend of polarization between males and females in groups."

More recently, the gypsy motif on the cover of "Little Queen" gave rise to rumors of the band's involvement in satanism and the occult, which prompts from Wilson a somewhat exasperated laugh. Exploitation in one form or another may very well be a legacy of Heart's make-up.

The group also has had to fight control of its own sound, and went to court in an attempt to terminate its relationship with Mushroom Records. Shortly afterward, Heart signed with Potrait and released "Little Queen," which without much critical praise or massive airplay has matched the sale total of "Dreamboat."

Mushroom then tried to release an album called "Magazine," which the group claims consists of unfinished tracks, and is trying to suppress. Test pressings sent out to critics three months ago showed the album, with less than 25 minutes of music, to be severely flawed. Heart managed to have several live cuts on the record restrained in court, and the result is that extant copies are already collectors' items.

"I hope it's never released, says Ann Wilson. "When there's a choice between money to be made as opposed to artistic ideals, the ideals are going to get shoved into the background."

Wilson, despite the problems, says she and the other band members are somewhat surprised at the suddenness of their success - a success that has rekindled her belief in the power and magic of rock.

"I see it a lot differently now," she explains. "When I first started into rock 'n' roll seven years ago, during the psychedelic times, I had this idea that rock 'n' roll was almost a missionary-type thing, where you took words and gave them as a gift to the people, as if that was going to save them."

The drudgery of those early years altered her perceptions for awhile, but "now I'm starting to regain that feeling of strength again where rock 'n' roll can be taken to the people as almost a therapeutic type thing. The only time when people can go and just totally relax and release themselves is at a rock concert. If doesn't matter what they look like or whether they scream and juump and down. It doesn't matter. It's rock 'n' roll."