Three new miniseries are introduced on Channel 7 tonight with episodes that suggest perhaps two of them could return to ABC next fall as regular fare.

One, "Eight is Enough," will stir a certain interest among Washingtonians although viewers familiar with columnist Tom Braden's affectionate book about raising his eight children with be hard put to find either the real Braden or his wife, Joan, much less the real-life children, in this series.

It debuts tonight at 8:30, through 9 p.m. Tuesdays will be its regular spot for the next five weeks.

"Eight" is a Lorimar Production and enough of the Sentimental but serious quality Lorimar gives to "The Waltons" comes through to suggest this series could make it.

But it does have problems. The "Tom Bradford" (Dick Van Patten) who appears in almost every scene tonight talks much too much, even for a Sacramento, Calif., newspaper columnist.

And the eight children, ranging in age from 21 to 3, are hard to sort out in this initial episode.

Moreover, the writers can't help injecting more than enough of that annoying familial democracy that is the hallmark of such shows. With a constituency the size of the fictionalized Bradfords, it gets a little shrill sometimes.

Nevertheless, the show shakes down into four almist-believable subplots - the oldest boy leaves home; a daughter is arrested on a drug charge; the youngest has a tonsillectomy; and Bradford finds himself in serious debt - which later episodes promise to unravel satisfactorily.

Friends of the Washington Bradens, incidentally, will find a non-careerist Mrs. Bradford, who is played with considerable strength by Diana Hyland.

"Three's Company," on the other hand, is billed as an "adult comedy." It airs at 9:30 tonight but will fill a regular Thursday slot for the next four weeks, preempting Tony Randall.

"Three" is almost total sexual innuendo - the old folks make jokes about impotence and the youngsters giggle about promiscuity and gays - but its randy charm is finally appealing.

Joyce DeWitt and Suzanne Somers play roommates who wake up one morning to find a homeless gourmet chief (John Ritter) in their bathtub.

Landlord Norman Fell doesn't like the idea of Ritter moving in, especially after his wife (Audra Lindley) thinks she's found the chef in drag. It's a lot more innocent than it reads and should appeal to several age groups.

Tonight's third entry, "Westside Medical," is strictly for nostalgia buffs. It is "Ben Casey" all over again - same sentimental plots, same irascible soul-searching by the young doctors.

Yet tonight's premiere at 10 does handle sympathetically a story of the trauman of the deaf who recover tteir hearing. But alas, "Westside" is a medical series that surrenders its promised authority to the overworked TV cliche and its future is doubtful. Unless it gets help - stat.