President Reagan arrived here today for the dedication of a museum honoring former president Gerald R. Ford, the man who defeated Reagan for the 1976 Republican presidential nomination and almost became Reagan's running mate last year.

Ford met the president at the airport on a chilly, rainy day and, in an echo of past Ford arrivals, the men bumped as they emerged from the door of Air Force One, causing Reagan to stumble.

Reagan's first business here, however, had to do with foreign policy, not the museum, which will be formally dedicated Friday.

The president held bilateral meetings with President Jose Lopez Portillo of Mexico and Prime Minister Pierre Elliott Trudeau of Canada who also came for the ceremonies opening the Gerald R. Ford Presidential Museum. Ford succeeded Richard M. Nixon as president in 1974 and was defeated for election to a term in his own right by Democrat Jimmy Carter in 1976.

White House officials stressed that Reagan had "no intention of upstaging Ford," but his meetings drew media attention from hoopla attending the Bob Hope Gala, almost an inevitability at such occasions.

In addition to the three heads of government, Vice President Bush, former secretary of state Henry A. Kissinger, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and numerous other politicians who worked with or against Ford over the years came to honor him.

Reagan warmed up for his neighboring heads of government by entertaining former French president Valery Giscard d'Estaing for 15 minutes.

Trudeau was next into the Reagan hotel suite, spending 45 minutes on what a senior administration official called the "very difficult, some would say intractable, problems" between the two nations.

Canadian anger about "acid rain" pollution crossing the border from U.S. industrial areas and U.S. concern about Canadian energy policy were among the difficult issues.

"Our objective is not to allow issues to fester," said the senior administration official who briefed reporters on condition that his name not be used.

Reagan has made good relations with Canada and Mexico a principal goal of his foreign policy and again today brought up his plan for Caribbean basin development. He is seeking support for the plan from Trudeau and Lopez Portillo.

"This was not a heavy business meeting," a senior administration official, dressed in black tie for the gala, told assembled reporters. "It would have been unusual if they the three presidents had not taken the opportunity to sit down together."

The senior official said Reagan has been seeking public and private money to help fund development in poorer Caribbean nations. In addition to his initial appeal to nations in this hemisphere, Reagan also has approached Japan, soliciting funds for a project Washington considers important to the entire world.

Lopez Portillo followed Trudeau for another 45-minute session with Reagan, much of it devoted to the upcoming 22-nation summmit that Mexico will host in Cancun late next month.

Although Reagan and Lopez Portillo have exchanged warm public gestures of friendship, they disagree sharply on policy toward El Salvador.

The senior American official said they discussed El Salvador, where the United States is arming government forces while Mexico favors a hands-off stance and recently joined with France to offer a form of recognition to antigovernment forces.

The Reagan-Lopez Portillo meeting was "extremely constructive," the senior official said. The Mexican president clarified the French-Mexican initiative for Reagan, and "differences were narrowed," the official said. The two also discussed future approaches to El Salvador that might be more compatible with Washington's thinking, he added.

The official said Mexico and France have been made keenly aware of Washington's displeasure with that initiative toward the Salvadoran opposition.

Secretary of State Alexander M. Haig Jr. was asked by reporters what he remembers best about Ford. Haig recalled: "I was the fellow who notified him that he was to be President Nixon's vice president." Haig added that he also had brought Ford word in 1974 that Nixon was going to resign.