With John Ritter, personality threatens to get out of hand. The star of "Three's Company" comes across in his pleasant and physical Sunday night ABC special as a Jerry Lewis with brains and a Chevy Chase with heart. He's an agile and engaging light romantic comedian whom television fits like a coat of paint.

But it's the age-old problem -- he needs better material. The sketches for "John Ritter: Being of Sound Mind and Body," at 10 p.m. Sunday on Channel 7, seem designed primarily to facilitate pratfalls, animal noises and the spitting of water by Ritter. One clings to the hope that there are resources not being adequately tapped here.

The best of the skits is the third, co-written by Ritter and Joe Landon, and it is also the one in which Ritter relates most directly and with the least amount of monkeyshines to the camera. He plays Walter Simmons, a man who has filmed pivotal moments in his life from 1925 on. It's touching and relatively subtle.

In other bits, Ritter is a klutzy partygoer trying to master body language, a movie theater manager who looks like Nixon and subdues rowdies with hero shtick from the silver screen, a man turned into a monster by too many trips through the airport X-ray machine, and a father torn between a rerun of "Rebel Without a Cause" and his baby's insistent crying.

Promising premises often go quickly flat, however, and the program is never quite as amusing as one wants it to be. Guest stars include Vincent Price, Howard Hesseman and, from "Three's Company," the chipmunky Joyce De Witt and the oafish Suzanne Somers. 'Hollywood Movie Girls'

Today's feminists would doubtless scowl at Jack Paar's famous introduction, "Here they are, Jayne Mansfield." And yet, has Ann-Margret ever really added up to much more than an attractive pair of breasts? They really are the featured attractions of "Ann Margret -- Hollywood Movie Girls," an ABC special that ranks with the season's most garish and egregious gar-bahge.

The inexcusably lengthy (90-minute) romp, at 9:30 tonight on Channel 7, is ostensibly a tribute to the glamor of Hollywood, but it manages that only by association. It's really just a narcissistic binge for the star, who looks cute in the opening film footage of her screen test (at the age of 19), pointing her breasts to and fro -- mostly fro. But she proves, again, pitifully shallow after a few more minutes' exposure.

Gary Smith and Dwight Hemion, the producers, know how to manipulate TV technology (they did "Baryshnikov on Broadway") but that's a hopeless task when you are throwing a party in honor of nothing.

The script, by Buz Kohan and Marty Farrell, is abysmal whether aspiring to humor (a silent movie musical with Dom DeLuise), pathos (an embarrassing "older woman" romance with male cupie doll Dean-Paul Martin) or simple fatuousness, as when Ann-Margaret says "Making movies is fun, but it's also a very serious business."

For a fleeting moment in a sketch with Roger Moore, Ann Margret is fairly funny as, in turn, Swedish, Hungarian and Italian women trying out for the part of Scarlett O'Hara. But then things get All Serious for a dumb call-it-a-day scene with Moore followed by Ann-Margret's shrieking and atonal rendition of, eek!, "I Will Survive."

You have to feel sorry for the lummoxes in Hollywood who imagine America will be enthralled by pap as drab as this.