It's a good thing NBC taped its third annual "Christmas in Washington" special early, because Christmas in Washington wasn't on the agenda of many of the performers and guests at the National Building Museum taping last night.

President Reagan and Nancy Reagan, who not only had front row seats for the show but put in an appearance, will be off to California for the holidays. So will Attorney General William French Smith ("It's traditional for the family"). Mezzo soprano Frederica von Stade, one of the show's stars, will be skiing in Italy, and emcee Hal Linden will hit the slopes in Utah.

But last night they had their minds on celebrating a Washington Christmas, with a lot of caroling as well as an appearance by NBC correspondent Roger Mudd, who pretended to be reporting live from the scene of Christ's birth.

After the president and first lady came on stage at the show's conclusion to wish everyone Christmas greetings, the Reagans joined the performers in singing "Hark the Herald Angels Sing" ("Actually, Nancy Reagan outsang me," Linden confessed at the party following the show). Joining von Stade and Linden were Nell Carter of NBC's "Gimme a Break," the Osmond siblings, the Shiloh Baptist Church Choir and the U.S. Naval Academy Glee Club.

After the taping, performers and guests walked behind the television backdrop to find clowns, mandolin players, jugglers, salmon, oysters, eggnog and, heaped under a huge tree, gifts brought by the guests for Nancy Reagan to take on her visit to Children's Hospital next week (NBC is also giving $25,000 to the hospital).

Asked at the party about reports on the storming of the Kuwaiti airplane in Tehran yesterday, Reagan said, "We're still trying to get confirmation." Secretary of State George Shultz, who was also at the party, refused to comment.

NBC chief executive Grant Tinker, when asked of the president's performance on the special, said, "We'd be hard pressed to find someone who can do it better than he did." Tinker seemed satisfied with the show. "This kind of thing "Christmas in Washington" kind of passes in the night and no one notices. But this show went pretty well." Of NBC's rise from its perennial third-ranked status in the network wars, he said, "We have a lot of the ambition needed. I'll settle for people just thinking that we're trying harder every year."

George Stevens Jr., creator and executive producer of the "Christmas in Washington" specials, said of his brainchild, "Sometimes you have an idea and it becomes what it should . . . When I started out, I wanted a joyous tradition of Christmas in the nation's capital to be enjoyed by everyone."

A wheelchair-bound James Brady was also at the show and party, displaying his characteristic wit. "I'm getting around a lot better," said the White House press secretary. Asked what his agenda this Christmas would be, he quipped, "Oh, getting around a lot better."