Three men who have filled top defense and national-security positions during the last 25 years say the Geneva summit meeting and arms-control talks present serious dangers and limited opportunities for the United States.

Former secretaries of defense Robert S. McNamara and James R. Schlesinger and former national security affairs adviser Brent Scowcroft gave the cautionary assessments of prospects for arms control and the meeting in November between President Reagan and Soviet leader Mikhail Gorbachev.

They made a joint appearance Saturday night at the American Political Science Association's annual meeting.

All three said the fate of the summit lies largely in the president's use of his Strategic Defense Initiative (SDI) as a key bargaining chip. None was optimistic.

Scowcroft, an adviser in the Nixon and Ford administrations and former head of a bipartisan commission for Reagan on the MX missile, said Reagan's emphasis on SDI, also known as "Star Wars," complicates U.S.-Soviet dealings and makes the NATO allies "extremely apprehensive."

Some fear that SDI's costs will divert funds and attention from the conventional defense of Western Europe, he said, while others think it will make obsolete the independent nuclear forces of Britain and France.

"The Europeans have always feared either U.S. co-optation or abandonment," Scowcroft said, "and SDI has a unique capacity to ignite both fears."

Schlesinger, who headed the Defense Department and the Central Intelligence Agency for periods in the Nixon and Ford administrations, said those fears virtually assure "that the Soviets are going to score a propaganda coup in Geneva and create more serious divisions in the alliance."

McNamara, defense secretary under Presidents John F. Kennedy and Lyndon B. Johnson, said the United States and the Soviet Union have switched positions since the 1967 summit in Glassboro, N.J., with Reagan adopting the "distorted" view then argued by the Soviets that "nuclear defense is moral and nuclear offense immoral."

The three officials differed on some prescriptions and assessments, but all said the United States should continue research on strategic defense to protect against what they called "a Soviet breakout."

The problem, McNamara said, is that administration plans for SDI "would have us violating the terms of the [1972] Anti-Ballistic Missile Treaty very soon," alarming soviets and Europeans.

McNamara said the Geneva talks need not be "futile" if "we take the president at his word and make . . . sure SDI is a research program and nothing more."

To do that, he said, the United States must agree to define research as "activity that is inherently unverifiable and say that anything that is verifiable [by intelligence means] is development, subject to the limits of the ABM treaty." Schlesinger, more pessimistic, said the administration has created the notion of SDI as "a mighty arsenal . . . . I see no hope of progress toward a serious, substantive agreement unless the president makes SDI negotiable."

He said the administration refuses to do that because Reagan and others "are reluctant to accept the inevitability of mutual vulnerability [to enemy nuclear attack] that has been the bedrock for arms-control negotiations since 1972."

Scowcroft, who said he saw more long-term potential in SDI research than the others if it leads to a world "with weaker offensive weapons and stronger defense," said negotiation of such a transition poses "a challenge of much greater magnitude" than the administration seems ready to undertake.

He said he worries that there are "at least five different concepts of SDI . . . and the administration has not by any means sorted out where it is going."