A key subcommittee is the powerful House Appropriations Committee Wednesday made cuts in the National Aeronautics and Space Administration budget that threaten the survival of three of its most ambitious plans over the next five years.

The three plans threatened by the moves of the House Appropriations subcommittee chaired by Rep. Edward P. Boland (D-Mass.) are upcoming attempts to search for life beyond earth, keep the Skylab space station from falling back to earth, and fly a rare joint mission with West Germany around the north and south poles of the sun.

"All three budget suts are terrific disappointments," said a Carter administration source who asked not to be identified. "We had fierce competition this year for new programs in the space agency and we felt that the ones that survived the competition were the best things we had."

What Boland's 11-member subcommittee did was to order a cut of $1.4 million in a request of $2 million by NASA to begin a program called SETI, which stands for the Search for Extraterrestrial Intelligence.

While the subcommittee did not tell the space agency it could not begin its SETI search, it in effect killed it by leaving the space agency with $400,000 to start the search. Said one NASA officials: "It's not enough to start design work on th antennas we were going to use to listen for extraterrestrial signals. It kills the program."

While the search for intelligent life beyond the earth has been criticized by some members of Congress, it has wide support from scientists around the world. It has been called the kind of program that could change the course of history, it successful.

The Appropriations subcommittee also denied NASA $20.5 million to place abroad the space shuttle in 1979 a device that would allow shuttle astronauts to affix an engine to the abandoned Sktlab space station to fire it up into a higher and safer orbit, where the 80-ton space station would stay for another 100 years.

The reason given by the subcommittee is that the shuttle's first flights mightbe delayed until so late in 1979 that teh astronauts mighr not be able to rendezvous with Skylab before the space station falls to earth. Space agency engineers say Skylab could plummet back to earth as soon as the fall of 1979.

The space agency has recharged Skylab's batteries before it attempts later this month to slow down Skylab's decent towards earth. Said one subcommittee aide: "If they're successful, some accommodations might be made."

No accommodations were mentioned when the subcommittee took $30 million out of three unmanned spacecraft projects and transferred it to the space shuttle budget as a "contigency funds." The subcommittee said it shifted funds to the shuttle to soften and impact shuttle delays might have on shuttle schedules in the next few years.

The $5 million cut in the $13 million request to tart the solar-poalr orbiter mission in 1983 is vewed as the most critical. Even a year's delay in funding this flight might mean a three-year delay in the mission, mostly because te positions of the earth and Jupiter will undergo an unfavorable change in 1984. The spacecraft is to get a gravity assist from Jupiter to fly on a path that takes it over the poles of the sun.

Two spacecraft are to undertake the mission, one built by the United States and the other by West Germany. The West Germans said a delay by the United States would mean Germany would drop out of the mission.