President Reagan returned from Camp David yesterday to make final preparations for his speech to a joint session of Congress at 9 tonight, his first public appearance outside the White House since he was shot March 30.

It will be Reagan's third speech since taking ofice devoted to economic problems and is designed to rally support for the program of federal support for the program of federal spending cuts and tax cuts that he has proposed.

Reagan and his advisers consider it vital for the president once again to put his personal stamp on the effort to sell his program after the pause at the White House caused by the president's injury and hospitalization.

The first Reagan economic speech, on Feb. 5, described the nation's economic problems. On Feb. 18 Reagan offered the programs he considers to be the best solution.

His third address, in a week when there will be key congressional votes on his programs, is an appeal for action.

The president wants to get across the message that "the day of decision is near," deputy press secretary Larry Speakes said.

Another White House official said the president will be telling Congress in his approximately 15-minute speech that "we're at the edge of the river. Do we wait nine months to build a boat or do we jump in and swim the currents?" The aide, after reading a draft of the speech, said Reagan wants members of Congress to be brave enough "to take the leap of faith" that supporting his programs requires of some.

Reagan wrote the speech's opening passage on a yellow pad and gave it to his chief speechwriter, Ken Khachigian, April 17. The president took a draft to Camp David on Saturday and spent some of the weekend editing and polishing it.

Reagan will include a mention of the assassination attempt that sent him to the hospital with a bullet in his left lung, but does not intend to officials who have read drafts of the speech.

The drama of the president's return to the House chamber less than a month after the shooting is calculated to focus maximum attention on his appeal for support. On the eve of the speech, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said many Democrats are defecting to Reagan's side and that the president's spending-cut program will be hard to stop.