The three-day, 36-orbit mission is scheduled to begin March 17 from the Kennedy Space Center at Cape Canaveral, Fla. Though a delay in the launching would not be a surprise because of the tight schedule for remaining preparations, John W. Young, the mission commander, reported that ''progress at the Cape is going super.''

Mr. Young and Capt. Robert L. Crippen of the Navy, the pilot, today held their last formal news conference before the mission after completing a successful rehearsal of their flight from liftoff to landing at the Johnson Space Center here. The astronauts worked round the clock in a computerized simulator, which they operated in conjunction with flight directors in Mission Control as they would the Columbia on the real mission.

The two astronauts have been in training three years for the oftendelayed first flight of the shuttle. They have spent more than 1,600 hours in the mission simulator and, as Mr. Young pointed out, that was more time in full-scale simulation than astronauts had in flight time during the entire 10-mission Gemini program in the mid-1960's.

''If there's a vehicle we can have confidence in, it's this one,'' Mr. Young said, answering questions about the risks of flying the first time in the trouble-plagued space plane. ''We obviously think it's safe.'' Agrees on Canceled Test

Mr. Young added that he agreed with the decision of the National Aeronautics and Space Administration to forgo a preliminary unmanned test flight, which had been the practice in previous manned programs. Such a flight, he said, could have added $500 million to project costs and delayed the program at least a year.

Captain Crippen said it was possible that flight problems could cut short the mission's three-day duration. He said that the least hint of important malfunctions would prompt a decision to return to earth immediately.

But Captain Crippen observed that ''going up and coming down are certainly the most critical phases of the flight,'' so that achievement alone would go a long way toward meeting most of the mission's objectives.

If this first shuttle flight is relatively flawless and there are no major repairs required on the huge winged vehicle, the plan is to send Columbia on its next mission in August. That would be the first time a vehicle has ever flown into and out of space more than once.

Next week the astronauts expect to continue simulation training and to test their pressurized space suits and supporting equipment that would be used in the event they have to go outside their cabin during the flight.