Senate Majority Leader Robert C. Byrd joined President Carter and national security affairs adviser Zbigniew Bezezinski yesterday in urging the Senate to avoid linking the fate of the strategic arms limitation treaty with the presence of Soviet troops in Cuba.

Byrd told reporters that the treaty had been "in rather good shape" after hearings ended in July but now would not get the two-thirds vote needed in the Senate for approval.

"But we're not voting on it today," he said. "All of this stampeding is what we need to avoid . . . The treaty could very well be in the interests of the United States, totally apart from the Cuban flap."

He said the presence of 3,000 Soviet combat troops in Cuba was "not comparable" to the 1962 crisis in which Soviet missiles were spotted on the island. "We don't know yet what their [the troops'] mission is," Byrd argued.

Confirmation of the troops came only recently, though they apparently had been on the island for several years, but Byrd said, "I'm not prepared to say it's an intelligence failure."

Telling Russians from Cubans in identical uniforms "probably requires eyeball to eyeball intelligence" that is difficult to get in Cuba, he said.

Brzezinski was more outspoken, telling out-of-town editors Friday that it would be "fundamentally prejudicial to our interests to link SALT II to Soviet behavior elsewhere. It really is, I think, self-defeating and if I may go even further than that, escapist.

"I think those who are saying, 'Let's hold out on SALT II,' I think they ate chickening out, frankly."

Transcripts of the interview were released yesterday. Shortly after Brzezinski spoke, Carter made the same point to the editors. "SALT II ought to be passed on its own merits," he said.

Peaceful competition with the Soviets will continue for many years in trouble spots around the globe, Carter said, "and without a SALT agreement limiting strategic nuclear weapons, each one of those differences . . . is much more likely to become a major threat to our nation's peace."

Brzezinski stressed that intelligence agencies had not spotted the Soviet troop presence earlier because "we weren't looking for it." Whatever the original reasons for the placement of 3,000 men there, he said, they may have changed by now. "Sometimes, even in a love affair, do you know the exact motives you are dealing with? You don't. You can only judge by actions," he said.

What is of concern to us is . . . the presence of an organized combat formation in the Western Hemisphere and in a country that is certainly pursuing very actively policies that are adverse to us . . ."

Byrd said he would try to bring SALT to the Senate floor by Nov. 1, by which time "the dust should have settled" from the troops fracas. Debate should not go past Thanksgiving, he said.

Asked by the editors about the future of ousted U. N. Ambassador Andrew Young, Carter reiterated his confidence in the man he fired and said, "I would welcome Andy in any major position." He speculated that Young will go to the private sector rather than stay in government and added, "He is very supportive of me and his voice will be heard."

Brzezinski, turning to the question of Cuban involvement in Nicaraqua, characterized the new Sandinista revolutionary government as "an amalgam of radical and progressive movement." Although "a significant wing" of the Sandinista movement was trained in Cuba, he said, "there are very different elements in it, some of which, doubtless, are very sympathetic to (Cuban premier Fidel) Castro . . . but some of whom are not."

"We believe that there is a possibility that the outcome in Nicaragua will be a government that is responsive to the plurality of Nicaraguan life," he said. To assume it will be pro-Castro, he said, "may end up in a self-fulfilling prophecy."

Byrd was asked to assess the political impact of Sen. Edward M. Kennedy (D-Mass.) being freed by his family to announce for the presidency next year. "He has given a signal that keeps his options open," Byrd said. He added that he presumed Kennedy would weigh the possible consequences to Democratic Party unity of a strong challenge to an incumbent Democratic president.

Carter is getting mixed reviews in West Virginia, Byrd said. "They give him good marks on foreign policy . . . (but) on domestic issues they don't think he has done so well." Byrd said those views are consistent with his own, but he noted that current issues are "so thorny, so difficult and so universal that it would be extremely difficult for any president to have high marks."

Byrd repeatedly has said it is too early to write Carter off for reelection because of the immense power of an incumbent and of Carter's strong image "as a good and honorable man."