Three senior American diplomats left suddenly on a mission to Turkey Monday night, taking new decisions by President Carter on the troubled relations between the two countries.

Deputy Secretary of State Warren Christopher, the State Department's No. 2 official, headed the surprise mission, which will also stop in West Germany and England before returning home. The latter two stops - but not the Ankara visit - had been previously blanned, State Department sources said.

The mission to Turkey was precipitated by decisions late last week on the part of Carter, who reportedly has given the diplomats a personal message to Turkish Prime Minister Bulent Ecevit. Official sources, who declined to be identified, indicated that Carter's decisions were in the form of U.S. proposals to be discussed with Ecevit and which therefore hinge to some degree on the results of the Ankara talks.

The Carter administration has promised to announced on April 6, before a House International Relations subcomittee, its position on the proposed $1 billion military aid program for Turkey that has been stalled for more than two years because of congressional disapproval of the Turkish occupation of northern Cyprus. Turkey has also been pushing for U.S. action to lift the congressionally imposed arms embarge that also stemmed from the Cyprus conflict.

Turkey, on the southeast flank of NATO, has been expressing growing impatience with its western alignment in view of the U.S. arms ban. Ecevit, who took power in January, declared publicly last week that his country is "at the threshold of a new decision" about the NATO alliance because of its economic difficulties and the U.S. stance.

Ecevit also said, in a speech in parliament, that a recent meeting in Switzerland with Greek Prime Minister Constantine Karamanlis resulted in an agreement between them on "the goals" to be pursued in resolving the Cyprus situation. However, the Turkish leader reacted strongly and publicly in recent weeks to remarks by Secretary of State Cyrus R. Vance that seemed to link U.S. Turkish military ties to progress in the Cyprus dispute.

Just such a link between Cyprus and U.S. military aid was the essence of the arms embargo passed by Congress in 1975 over the strong objections of the Ford administration. Pro-Greek forces on Capitol Hill have insisted that major aid to Turkey be conditioned on concessions by that country on Cyprus. Carter endorsed this position while campaigning for the presidency but has been backing away from its since taking office.

An aid package for Turkey in the absence of progress toward a resolution on Cyprus is certain to produce a battle in Congress. The Carter administration is particularly skittish about such a fight right now because the final Senate vote on the Panama Canal treaty might be affected. Early last year, several influential lawmakers threatened to oppose the canal treaties if Carter went ahead with a plan to sell F4 jet fighters to Turkey. The administration backed down.

The suddenness and high level of the U.S. mission suggested that Carter's new decisions are not likely to satisfy the Turks. On the other hand, Washington sources indicated that opponents of new aid to Turkey may not be happy with them either.

Traveling with Christopher to Turkey were George Vest, assistant secretary of state for European affairs, and Matthew Nimetz, State Department counselor. The U.S. officials arrived in Ankara late yesterday, wire services reported.

Sources here said the stopover in Bonn on the way back from Ankara is in part to discuss the future of the neutron bomb, which is currently before Carter for decision.