For the third time, shuttle commander Robert L. Crippen will try Saturday to bring the 100-ton spaceliner Challenger back to Florida's Kennedy Space Center.

Bad weather forced him twice in the last 18 months to be waved off to California's Edwards Air Force Base. But this time, Crippen has been given a forecast of clear, sunny skies for the landing at 12:26 p.m. EDT.

So anxious is NASA to have a landing in Florida and get Challenger ready for its next flight in December that at least two mission rules may be bent. A landing in California can cost as many as seven days of work to get the shuttle ready for its next Florida launch.

One rule likely to be bent is the crosswind limitation, which prohibits landings in crosswinds of more than 8 knots. Today, northeast winds of 12 knots, gusting to 18, were measured at the three-mile-long shuttle runway, and the forecast for landing time Saturday was for northeast winds of 10 knots, gusting to 15.

"These northeast winds are not direct crosswinds; they're about 50 degrees off the crosswind center," Capt. Arthur Thomas, Air Force weather officer, said today. "Nevertheless, they're right at a crosswind limitation."

An informal rule on soaring birds may also be bent. Shuttle officials do not like to land in the heat of the midday when vultures, buzzards and hawks high in the sky could pose a serious hazard to the returning space shuttle. In its only previous landing here, the shuttle ran into a flock of birds, damaging an engine.

"We haven't changed our minds about landing in the heat of the day, but we think we have a situation here we can handle," shuttle landing director Robert Webster said. "This time of year, we don't have any migratory birds down here to worry about, and we think we can cope with our local birds as long as we're aware of their numbers and locations at landing time."

A half hour before the landing, a pair of NASA helicopters will sweep the area around the runway to get an idea of where the birds are, Webster said. If soaring birds are out in any numbers, Cape personnel will drive trucks along the runway and blow their horns to chase the birds away. Security guards will fire shotgun blanks to create still more noise.

Meanwhile, the crew of Crippen, Jon A. McBride, David C. Leestma, Kathryn D. Sullivan, Sally K. Ride, Paul Scully-Power and Canadian Marc Garneau were busy wrapping up the last of their in-flight experiments and stowing equipment.

Working inside the cabin, Sullivan and Leestma transferred 134 pounds of toxic hydrazine between the tanks in 52 minutes, close to the projected 133 pounds in 53 minutes.

The only interruption came just before 12:30 p.m. EDT when President Reagan phoned the crew, as he has done on 11 of the 13 shuttle missions.

From his campaign whistle-stop train passing through Dayton, Ohio, Reagan reminded the astronauts that Dayton's Wilbur and Orville Wright started it all by building their first airplane. Reagan said it took the Wright brothers seven years and $1,000 to build the Wright Flyer while NASA took 10 years and $10 billion to build the shuttle.

"Challenger flies a little farther and maybe a little faster, and I suppose that justifies the slightly higher development costs," he said.

Reagan chatted briefly with Sullivan, Ride and Garneau before ending a call often interrupted by background noise. He asked Sullivan how her Thursday spacewalk was. Replied the first American woman to walk in space: "It was far more than I could have expected. It was the most fantastic experience of my life.