President Reagan invited 15 political cartoonists from all across the country to lunch at the White House yesterday, but noticeably missing were three of the president's toughest critics: Herbert Block of The Washington Post, Paul Conrad of the Los Angeles Times, each of whom has won three Pulitzers, and Garry Trudeau of "Doonesbury" fame, also a Pulitzer Prize winner. They were not invited.

The president's communications director Pat Buchanan, who drew up the invitation list, said no one was blackballed and that "no offense should be taken in any way" by noninvitees. He said many outstanding cartoonists in the country were not asked because the luncheon could only accommodate 15. As for "tough" cartoonists, he said several on the White House list "are as tough or tougher" than Herblock, Conrad and Trudeau. Trudeau, who usually does not comment, said yesterday, "If I were he [Reagan] I wouldn't have invited me either. But I'm surprised he didn't invite Herblock, the dean of American cartoonists."

Deputy Press Secretary Pete Roussel said the luncheon was especially congenial, with the cartoonists drawing caricatures of the president and the president himself even going to the drawing board to draw a cartoon. The political cartoonists proved to be an observant lot. Several discovered that the president's barber has been clipping away at his famous pompadour, long a favorite target. When the question about his new hair style was raised, Reagan conceded the barber "cuts off the wave."

Among those at the lunch were five Pulitzer Prize winners: Jeff MacNelly, Chicago Tribune-New York News Syndicate; Bill Mauldin, Chicago Sun Times; Pat Oliphant, Universal Press Syndicate; Mike Peters, Dayton Daily News; and Paul Szep, Boston Globe. White House aides attending were chief of staff Donald Regan, national security affairs adviser John Poindexter, Buchanan and Roussel.

Heston's Biggest Fans The president and First Lady Nancy Reagan will be in the audience tonight to see their old friend Charlton Heston make his Washington theater debut when he opens in Herman Wouk's "The Caine Mutiny Court-Martial" at the Kennedy Center's Eisenhower Theater. After the performance, the Reagans will go backstage to meet with the actors in the Green Room. British actor Ben Cross, a star of the hit movie "Chariots of Fire," is also making his Washington debut.

"The Caine Mutiny Court-Martial," which played in London last year and, recently, three weeks in Los Angeles, had a preview performance last night followed by a party for Heston hosted by Frank Hodsoll, chairman of the National Endowment for the Arts, and his wife Mimi. Among their guests at the dinner at the Watergate Hotel were Washington author Wouk, his wife Betty and their son Joe Wouk, a producer of this production, and his wife Suzanne.

Other guests included William F. Buckley and his wife Pat; George Will and his wife Madeleine; American Film Institute Chairman George Stevens and his wife Liz; Heston's wife Lydia; and Lynne Cheney, the new chairman of the National Endowment for the Humanities. End Notes

The trial of Cathy Evelyn Smith, charged with second degree murder and 13 drug counts in the 1982 drug overdose death of comedian John Belushi, was delayed yesterday in Los Angeles Superior Court. Smith reportedly refuses to sign any agreement that will mean a jail term for her. The prosecutors, however, are adamant that any deal must include some time behind bars . . .