Three sides of Ronald Reagan were revealed last week in separate instances in which the president wisely vetoed a premature campaign trip, displayed ignorance about his own education program and surprised congressmen with an old actor's appreciative review of a new movie.

Reagan has always believed that all work and no play makes for a dull president. He went to Camp David last weekend with a briefcase full of option papers on arms control but found time to preview "War Games," a film in which a teen-ager with more flair for computers than formal schooling ties into the North American Air Defense Command computer and almost launches World War III.

On Tuesday, a day before he announced his revised proposal for a nuclear-arms treaty with the Soviets, the president briefed a group of moderate House members who had voted for the MX missile after extracting a promise from Reagan to be diligent in pursuit of arms control.

"I just can't believe that if the Soviets think long and hard about the arms race they won't be interested in getting a sensible agreement," Reagan told them.

But after this satisfying declaration, he startled his audience by relating how he had screened "War Games." As the president started to describe the movie, one congressman said, "his face lit up." Reagan became so engrossed in relating the plot that one congressman even said to him: "Don't tell the ending."

Some congressmen were struck by the incongruity of a president of the United States becoming more absorbed in a movie about narrowly averted nuclear war than in the real-life possibilities for arms control.

"It was really funny," one participant said. "I was sitting there so worried about throw weight, and Reagan suddenly asks us if we've seen 'War Games.' He was in a very good humor. He said, 'I don't understand these computers very well, but this young man obviously did. He had tied into NORAD!' "

Continuing with his impromptu movie review, Reagan found "a little bias" in the casting of the high school teacher in the film as "a wimp." Then he turned to Army Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, and said with a smile, "They portrayed the general as this slovenly, mean, unthinking guy."

Reagan worked as a Hollywood actor for 15 years and has always been reluctant to distinguish movies from real life. On Nov. 21, 1975, during the opening week of his campaign against President Ford, Reagan treated reporters to a stirring account of how a black steward had taken over a machine gun from his fallen comrades during World War II, and he claimed that this act of movie heroism led to the desegregation of the armed services.

That would have been news to President Truman, who ended desegregation with an executive order three years after the war was over.

But it is not a bad thing to have Reagan watching "War Games" and approving of its message, if we judge our presidents' intentions by the entertainment they enjoy. Derring-do preferences that come to mind are President Kennedy's fondness for Ian Fleming's James Bond novels and President Nixon's for the movie "Patton."

"War Games" is an anti-war movie of the video-game era and a celebration of human values. Its message, stated by an advanced computer, is clear: The only way to win the "game" of global thermonuclear war is not to play it. That is a good idea for any president to have in his head.

Less can be said about Reagan's performance in Minnesota last Thursday. The president was not up to the politically motivated education offensive concocted by his advisers and forgot many of the basic elements in what is supposed to be the administration's education program. When he couldn't remember his education proposals, Reagan turned to Education Secretary T.H. (Ted) Bell to bail him out.

This, too, was reminiscent of earlier moments in the Reagan political career. On March 14, 1967, when he was governor of California, Reagan was asked by a reporter what his legislative program was, and he turned plaintively to his aides, saying: "I could take some coaching from the sidelines, if anyone can recall my legislative program."

In the old days, when Reagan goofed, his aides often understood that reporting of such lapses was necessary and inevitable. Now, they tend to draw attention to the president's miscues by criticizing those who report them. When NBC's Chris Wallace accurately observed that Reagan had done more damage to his education program by forgetting it than former vice president Walter F. Mondale had done in denouncing it in Minnesota the same day, White House spokesman Larry Speakes dutifully rebuffed Wallace. But the problem was the president.

On political matters, however, Reagan's instincts remain more secure than those of many of his advisers. The great White House game plan for July 4, when Reagan will be in California, was to send him home via New Hampshire, traditional site of the nation's first presidential primary. This was supposed to be such an unmistakable clue that even skeptics such as this reporter would proclaim that Reagan is a candidate for a second term.

But the staff's eagerness was not shared by Reagan, who learned about the plan from news accounts and promptly vetoed it. The president understands that a trip to New Hampshire at this time would raise expectations he is not yet ready to fulfill.

Stand-in Reaganism of the Week: Education Secretary Bell, in Minnesota: "I am very grateful that we have education up where it's high on the education agenda of this country."