One of the most thrill-packed half hours in television never gets on the air. It's the panic-stricken 30-minute countdown to 11:30 p.m. for the cast and crew of NBC's "Saturday Night Live," and for the third-season premiere last Saturday, things were running true to form: namely amok. Chickens with heads cut off would scarcely be noticed in these surroundings.

While John Belushi stewed that he couldn't get into his own dressing room an guest host Steve Martin ran through the crowded halls with his unbuttoned shirt flapping and the usual gaggle of fanatics tried to bluff their way into the show without tickets and the studio band revved up to full fever pitch, producer Lorne Michaels and fellow writers were one flight up on the ninth floor of 30 Rockefeller Plaza editing and rewriting the show that would start, live, in a matter of minutes.

Michaels, at 32 the late-night Orson Welles of latter-day television, thinks the third year will be the best for this trend-setting satirical theater of the air. But the concedes it may also be the last - at least for "Saturday Night" as we have come to know and love it. Michaels' contract expires at the end of this season, and though he wants to stay in TV and at NBC. "I can't work 18 hours a day for the rest of my life or I'll die."

In fact, Michaels, also one of the show's writers, is the man who got it on the air over extreme network skepticism "They were expecting 'Up With People,'" he once said, and maintains the delicate balance of egos and talents that goes into each program. With his depature, there would be the distinct danger of implosion.

Then, too, though unknowns when recruited, the Not Ready for Prime Time Players have since become household pests, and names, and all are getting offers that pull them in other writing and performing direction. Belushi is even now commuting between New York and Mexico, where he's appearing in a movie with Jack Nicholson.

"It's sort of like the fifth year of college, you know?" Belushi said last week. "The fourth year - great. But the fifth year, you feel like you're hanging around and you don't really belong."

Dan Aykroyd, Gilda Radner, Laraine Newman, Garrett Morris, Bill Murray and Jane Curtin are getting too big for the show that made them stars. It is not likely TV's foremost stock company will remain intact after this year.

They won't be leaving a flop, that's for sure. The show has proved a ratings smash, opened up network time previously dormant (commerical time is virtually sold out for the year ahead), extended the constituency of television to a young, educated audience previously ignored, altered Saturday nigh social life for millions, and given the NBC peacock one of its few excuses to crow.

As if to confirm that, the top NBC brass were on hand for Saturday night's third-season premiere, including new network TV president Robert Mulholland and chief programmer Irwin Segelstein, who lurked backstage trying to snare comic Martin - whose career skyrocketed after his "Saturday Night" stints - for a series of specials.

Mulholland himself intervened to restore one of the most brilliant bits on the show, Alan Zwibel's "Trinity 3,000," about computerized confessions, NBC censor Herminio Traviesas, the original Doctor No, had nixed the sketch during rehearsals. Michaels appealed to Mulholland and Traviesas was overruled.

Other glittery faces in the studio audience Saturday night reflect the show's singular status among television programs: Mick Jagger, Eric Idle of "Monty Python" fame, James Taylor and Carly Simon, and either Daryl Hall or John Oates - you know, the short one, who looks as though his mustache grew him.

Less glittery but surely at home was Lloyd Dobyns, the pocketsful-of-wry guy who hosts the NBC News "Weekend" show, seen once a month in the "Saturday Night" time slot. Dobyns brought his 17-year-old son, "a real fan of the show," to see it in person. Dobyns said he likes "Saturday Night," too. But Michaels confesses he has never seen "Weekend" in his life. He's been busy.

Michaels, who has advanced or regressed from last year's camel boots to this year's penny loafers, says he made a speech to th staff at the opening of the season that there would be no repeating of old bits. Hence we have seen the last of Baba Wawa ("unless Barbara Walters does something so extraodinary we can't ignore it") and even those from the planet Remulae and last year returned to it inside the Chrysler Building.

What will we see? A new set and a new opening, unveiled last week, an Update "news team" for the weekly mock newseast, recurring regulars like writers Franken and Davis and British comedian Barry Humphreys to relieve the regular regulars. Guest hosts may include Hugh Hefner for a special "them show," Michaels says, and he also notes, "I would love Bob Dylan to do it."

He also plans to go on the air himself the next show (Oct. 8) and announce an "Anyone Can Host" contest. The winner - whoever best articulates in 25 words or less why he or she would be the perfect guest host - will get to host the Dec. 17 Christmas show.

"You'll be sor-ree." Michaels is told after he mentions this grand scheme in his freakishly cozy 17th-floor office. But when you say something like that to Michaels, he only gets all the more tantalized. If he didn't like living dangerously, "Saturday Night" would not be the magnificent, nutty, rock 'em, sock'em boon to mankind that it is.

The whole real point of doing the program live is to live on the edge of doom. "The thing I can't give up is still the most exciting thing about the show. The best television of the past 20 years has been news and sports, because it's been live, too."

It is hard not to feel the pull of this challenge when sitting in the audience as the show goes on, those big fat RCA cameras on wheels just missing the other big fat RCa camera on a crane as they circile and regroup. During Steve Martin's opening monlogue, the crame camera descended to within one off of a spectator's head.

Three sketches were cut while the show was actually on the air. One of them was Michels' contest piel another was "Benjamin arrison: The Only 31 Days," a novel for television" about a U.S. President who caught pneumonia at his own inauguration an proceeded to die a few weeks later. Michaels not only supervises the cutting, he has to soothe the cuttees. Late Friday night he was comforting Gilda Radner by telephone because one of her routines has been scissored during rehearsals.

Why do they do it - these crazy zany kids? Well, they all got raises this year, and the show's weekly budget has gone up from about $150,000 the first season to about $250,000 now. But it would be lunatic naivete to think they're in it for the money. That's like saying the allies stormed Normandy for the exercise.

Partly it may be that the excitement of rediscovering real television is as energizing for them as it is for viewers. "Saturday Night" has considerably broadened the scope and sharpened the bite of TV humor (and is widely if feebly imitated), but its real contribution could be in taking TV back to its live real-tme, you-are-there origins.

The most frequent criticism of the show - that it's "uneven" - thus becomes meaningless. Der Ring des Nibelungen is uneven too. "But when it hits,t" says Michaels, sitting in his office, "then it's the best thing, ever!"

In other words, his decision to leave the show after this season is anything but irrevocable.