Noel Coward's Tonight at 8:30 is constantly popping up on playbills. But when you buy your ticket you don't know what you're going to see, for Tonight is a nonet of one-acters from which any given production is supposed to choose three. Unfortunately, the nine are of uneven quality; and so, perforce, will be the 84 possible shows.

This week's show includes three plays each requiring four women and five men, so that every performer gets a crack at playing three different people. The starring roles are taken by Faye Emerson and Murray Matheson.

The two stars do well indeed in the final play, Ways and Means, one of the best of the set. A bedroom comedy, complete with burglar, about a pair of upper-class house guests, out of funds, whose hostess wants to get rid of them, it is consistently funny. But why do they omit the final line? Without it, the end falls flat.

The curtain-raiser is Hands Across the Sea, a plotless bit of mayhem in which the lady of the house doesn't want to admit that she can't identify all her drawing-room callers. Too much time is spent talking on the telephone, and the device of having two or three people talk at once is vastly overworked. It is a three-minute joke extended to thirty.

The middle item, Shadow Play, is a confused, stylized soap opera about a marriage on the rocks. It uses flashbacks, flashaheads, and flashbetweens, with songs and rhymed couplets tossed in, if you please. It shows that the two stars ought not to sing in public; but it does provide a good final examination for lighting technicians and stagehands.