Suddenly it's a three-man debate. By his interview with an American journalist, Soviet leader Konstantin Chernenko has suddenly put himself - and the Kremlin - square in the middle of the upcoming face-off between President Reagan and Democratic challenger Walter Mondale. Diplomatic experts in and out of government see these Soviet motives behind the unusual move:
* It seeks to put pressure on President Reagan to respond positively to Soviet overtures for progress on arms control, even though there is no substantive change in the Soviet position.
* It prepares the ground for the post-election period and possible progress on arms control in the next administration, whoever is elected president.
* It shows that Mr. Chernenko, who has been ailing, is in good health and functioning well and that the Politburo is not weakened because of an aging leadership.
The Soviet President, in a 20-minute interview with Washington Post correspondent Dusko Doder on Tuesday, suggested that there could be an improvement in US-Soviet relations if Washington showed interest in reaching agreement on at least one of four Soviet arms control proposals. These proposals , spelled out separately in answers to written questions, include the proposal to prevent the militarization of outer space, a mutual freeze on nuclear weapons , US ratification of test-ban treaties, and a pledge by the United States not to be the first to use nuclear weapons.
The Reagan administration said it is pleased ''with the positive tone'' of Chernenko's comments. But a White House spokesman Wednesday reaffirmed US objections to the Soviet proposals, and in reply to the Soviet leader's call for ''deeds, not words,'' said: ''We agree. When the Soviet Union is prepared to move from public exchanges to private negotiations and concrete agreements, they will find us ready.''
There is nothing essentially new in the Soviet proposals, which were believed to have been raised by Soviet Foreign Minister Andrei Gromyko in his recent meeting with the President. Many analysts thus see the Chernenko ploy as timed to the presidential debate on foreign policy on Oct. 21 and aimed at warming up the climate and putting pressure on the next administration by setting the agenda.
''Nothing has changed in his position, and they will not do anything until after the election,'' says one State Department expert, ''but they do want to ease the atmosphere.''
The Kremlin prefers Mondale in the American election, and may now be encouraged about his electability, given the President's recent slippage in the polls. But most experts think the Soviet leadership remains persuaded that Reagan will win on Nov. 6 and is maneuvering in expectation of doing business with him. ''They're putting pressure on Reagan to say 'yes,' and then after the election they'll take him up on it,'' says William Hyland, editor of Foreign Affairs magazine and a Soviet specialist.
More interesting to administration officials and outside observers than Chernenko's statements is the mere fact of his appearance with a Western journalist. There have been reports that the Soviet leader appeared shaky in recent public appareances.
Mr. Doder described the Soviet leader as looking fit and in good health. ''His complexion was ruddy, his handshake firm and his gait steady,'' he wrote in the Post Wednesday. Chernenko conducted the interview without consulting his advisers or using notes, said Doder, and ''his delivery was far better than in his public speeches, employing plain language and normal Russian phrases that were in sharp contrast to the stilted formality of Pravda editorials.''
The Kremlin got out the message it wanted, say experts, both for internal consumption and the outside world. ''This was a clear move by the Soviets to take the wind out of the argument often advanced that no business can be done with them because their leadership is infirm and in disarray,'' says Mark Garrison, head of Brown University's Center for Foreign Policy Development and a former US diplomat.
Malcolm Toon, a former US ambassador to Moscow, says he believes that Reagan may have difficulties responding to debate questions about some of the Soviet proposals and that the Chernenko interview was aimed in part at influencing West European opinion. There is no good reason why the US has not ratified the nuclear test-ban treaties, he suggests, and the proposed talks on antisatellite and space weapons were handled ineptly by the President.
''The administration should have accepted the Soviet offer for talks and then brought up whatever it wanted when they began,'' says Ambassador Toon. ''The fact is, the administration did not want to tie its hands on SDI (Strategic Defense Initiative, a space-based system designed to defend against ballistic-missile attack) - which no one thinks is feasible - and so it threw in conditions on discussing nuclear weapons as well.''
Administration and other analysts, on the other hand, believe the President can handle the issue fairly easily. He has simply to say that he is delighted that the Soviets are coming around to a desire to negotiate, and that this proves the correctness of his defense buildup and arms policies.
''I don't think he's that easily touchable on the subject,'' says a State Department official.