About the most lavish praise that can be heaped on "The Dick Cavett Show" is that it's better than nothing; it does make the PBS schedule a little less gray and oblivious. A three-part Cavett session with a few of TV's top journalists, taped Oct. 12 and to be seen starting tonight at midnight on Channel 26, has moments of engrossing analysis, crusty badinage and sufferable bluffing.

David Brinkley, Harry Reasoner, Ted Koppel and Robert MacNeil are the guests -- Brinkley and Koppel piped in from Washington, framed and hung in Cavett's New York studio like a painting, something that might appeal to the people who buy portraits of dogs playing cards. Brinkley, by virtue of his seniority as well as his stature, should have been accorded more air time, but Cavett fails to call on him, and Brinkley doesn't leap in much.

When he does, he ventures such opinions as Vietnam was "the first uncensored war we ever had," which triggers, in part two of the series (tomorrow night at 11:30), a thoughtful if not particularly timely discussion of how TV covered the war, and how much TV coverage contributed to the U.S. decision to pull out. MacNeil says that for all the talk of how TV brought the war home, there was "a great deal of sanitizing" of war footage shown on the network newscasts. He also confesses that he went "too far" in letting his bias against the war color reports he did at the time for the BBC.

Discomfortingly -- whatever one may think of the merit or error of that pitiful war -- no one comes right out and says it wasn't the proper role of TV news to encourage or discourage American participation in it.

Now and then the discussion moves enticingly into the rhythm of a good barroom seminar, but Cavett is always there to deflate momentum; does he fear that an action-packed verbal donnybrook will deflect too much attention from him? He's so counterproductively polite that everyone tends to join him on tippy-toe. Still, there are little peeps of candor. Koppel says Dan Rather is "clearly overpaid." MacNeil says the opening of "Nightline," Koppel's show, is too "cluttered."

But Brinkley is being disingenuous, surely, when he pleads innocent of being a "star" in the public's eye and says, "We simply happen to be well-known." After 38 years at NBC and only a month or two at ABC, Brinkley can be forgiven still speaking of NBC in terms of "we." However, Reasoner cannot be forgiven for saying late in the last show that he and Howard K. Smith, not Roone Arledge, deserve the credit for ABC's rising status in the network news race.

Koppel's problem on a show like this is that he is either incapable or unwilling to let his guard down; the talent that serves him so well in the "Nightline" interviews may be his undoing in a situation like this. His eyes find and cling to the camera lens, he pontificates as if before a classroom full of high-schoolers, and he more than the other panelists appears preoccupied with how he's coming across to The Folks At Home.

Cavett says that CBS and NBC refused to let key journalists participate and that Reasoner only slipped through on "a technicality": He has a book to plug. This is further indication of what, unhappily, NBC News president William Small and CBS News president William Leonard have in common. They feel that what they do is beyond reproach and that they are accountable to no one.

Channel 26, meanwhile, earns another booby prize for its bulging trophy case by scheduling the three parts of this special Cavett show at different times each night (the Wednesday finale will air at 11), making it as difficult as possible for people to watch. At WETA, they've got the Midas touch, in reverse.