President Reagan is studying three proposals for changing the U.S. negotiating position at the strategic arms reduction talks (START) in Geneva, congressional sources said yesterday.

One plan is a variation on the so-called build-down idea in which each side would take away old weapons whenever adding new. It calls for annual reductions in the destructive power of each side's long-range or strategic nuclear forces.

A second alternative mainly would alter the warhead and missile launcher ceilings in the present U.S. proposal to bring them closer to what the Soviets have in mind. The third would make only cosmetic changes in the present START proposal.

Reagan has been under heavy pressure from Capitol Hill and some from Europe to modify his original START position along lines thought more likely to result in an arms control agreement.

More such pressure was put on the White House yesterday as six key members of Congress, including Sens. William S. Cohen (R-Maine), Charles H. Percy (R-Ill.) and Sam Nunn (D-Ga.) and Reps. Les Aspin (D-Wis.), Albert Gore Jr. (D-Tenn.) and Norman D. Dicks (D-Wash.) met with William P. Clark, the president's national security affairs adviser.

The six were leaders of a group of moderates who provided key support this May for the president's MX missile in return for his pledge to move with greater speed on arms control. Sensing that the administration may not be doing all it promised, they presented Clark with what one described as "eight principles" which they want adopted in a revised U.S. position in Geneva.

If they are not adopted, one participant said, Clark was told the group "would not support the MX program" when it comes up next month for approval of production funds.

While this jockeying continued over START yesterday the administration said it already had modified its proposal in the companion talks in Geneva on intermediate-range nuclear forces (INF). In a question-and-answer session with regional editors and broadcasters, Reagan said he had sent chief U.S. negotiator Paul H. Nitze new initiatives to be presented to the Soviets.

White House spokesman Larry Speakes refused to spell out what these are, but sources said Reagan would drop his earlier demand that the United States and Soviets have the same numbers of medium-range missiles worldwide, and settle for such equality in Europe.

In Asia, the Soviets would be allowed to keep their present number of 108 SS20 missiles without an equal U.S. force, but only in return for a guarantee that the Soviets would not increase their levels.

In announcing he had made his INF changes, the president said, "The time has come for the Soviets to show the world that they're serious about peace and good will."

On START, one participant in yesterday's session said the members of Congress would be given the president's general reaction today and some specifics "within a few days."

Reagan is expected to outline his new arms control positions when he addresses the U.N. General Assembly next week.

In the administration's internal debate over the START proposals sources said the Pentagon and some conservatives in the White House favor few if any changes, while at the State Department there is a desire to move closer to the Soviet proposals.

In these internal councils support for build-down comes primarily from the so-called Scowcroft Commission the president named earlier this year to review the administration's nuclear arms policy. Much as it did during the MX negotiations earlier this year, the commission has become a broker between Congress and the administration.

Retired Lt. Gen. Brent Scowcroft, chairman of that commission, was present at yesterday's White House session.

The six legislators decided to press for the meeting because they felt they had to show "there was unity between the two bodies on the connection between MX and arms control," one aide said yesterday.

They needed to present a "solid front," he added, because the members saw the White House was "trying to cut deals" with different members.

As one aide put it, "They felt they were being jerked around by the White House.

"We were told some insiders within the administration were treating it more as a congressional management problem than a policy issue," the aide said.

The three senators have been waiting, with growing impatience, for the White House to fulfill Reagan's May commitment that in return for their votes on the MX, he would include build-down in a revision of his arms control proposals.

The administration's plan was first promised Percy in mid-August, and was used by the Foreign Relations Committee chairman as an excuse to postpone a requested vote on the nuclear freeze resolution.

In late August, it was announced it was delayed until September and a final promise to deliver a plan on Sept. 20 was given by Clark in a letter to Sen. Larry Pressler (R-S.D.).

On Tuesday, with the plan not in hand, a resolution suporting build-down sponsored by Percy and Pressler failed to pass the Foreign Relations Committee.

The build-down proposal already being considered by the White House is similar to that which has been pushed by the group, the source said.