Three arms control specialists outside the government gave the first concerted support yesterday to the Carter administration's nuclear arms limitation negotiations with the Soviet Union.

This first significant support from the "dovish" wing of the arms control community included an admonition against claiming too much for a controversial accord still under negotiation.

"I think that there is a real of the administration oversalling this agreement," said Jan M. Lodal, former director of program analysis for the National Security Council under Henry A. Kissinger. He spoke at a panel discussion sponsored by the Arms Control Association.

In its present form, Lodal said, the agreemnet "would get a B-minus in my book." But, he said, "rejection of this treaty by the Senate would be an absolute disaster," for "the treaty is not so bad that it deserves rejection even if all of Paul Nitze's worst fears come to pass."

Nitze former deputy secretary of defense ans a former U.S. negotiator in the strategic arms limitation talks (SALT), is leading the non-governmental attacks on the administration's proposed terms with the Soviet Union. Sen. Henry M. Jackson (D-Wash.), trol Subcommittee, is the leading challenger in Congress.

"There is a whole lot of logic missing" from the claims raised by the Nitze-Jackson camp about the vulnerability of the American land-based intercontinental Minuteman missile force, dsaid Richard L.Garwin of the Thomas J. Watson Research Center.

Minuteman is only "one leg of the triad" of America land, sea and air capacity in strategic weaponry, Garwin noted.

While the Minuteman force will become vulnerable to Soviet attack in the 1980s, Garwin said, "Minuteman is not all that vulnerable" that the Soviets can assume they can eliminate it. To attack the 1,000-missile force, Garwin said, would expose the Soviet Union to massive retaliation by other U.S. weapons.

If arms control means anything, Garwin said, "We have to start from the view that Soviet military people are rational. . . "

Herbert Scoville Jr., a former scientific adviser in the Defense Department, Central Intelligence Agency and the Arms Control and Disarmament Agency, gave the strongest support to the administration's case.

"I also do not think this is the largest agreement in the world," Scoville said, but "a failure to get this ratified, I feel, would be a great disaster for our security as well as for our pocketbook."

Although the proposed nuclear arms cuts do not go far enough, Scoville said, "the Soviets are the ones who are going tro have to cut, while we don't have to cut at all."

The proposed ceilings would limit American and Soviet armed forces to a total of either 2,160 or 2,250 strategic missiles and bombers on a side. The Russians today have about 2,500 strategic weapons, the United States "about 2,130," Scoville said.

All of the panelists agreed under wquestioning from reporters, however, that the administration's challengers have the public advantage of simplicity in attacking the intended accord, because of the complexity of the issues.

The administration is further handicapped by its refusal to discuss numbers and other details publicly which have been "leaked" to the press.In addition, in the administration's more ambitious plan for makor arms cuts which the Russians spurned last March, some officials, notably presidential adviser Zbigniew Brzezinski, emphasized the U.S. intention to limit Soviet threats to the Minuteman land-based missile system.

No proposal yet made by anyone, Scoville said, including Sen. Jackson, would fail to make "Minuteman vulnerable on paper," Nevertheless, and knock out 1,000 Minuteman simultaneously is almost a pipe dream."

Lodal said that the argument that is "implicit" in many demands of the administration's severest critics "is that we really shouldn't continue the SALT process." He said "I think people should really come out say this honestly."

Sen. John C. Culver (D-lowa) and four other Senators yesterday made a formal request to Sen. John C. Stennis, chairman of the Senate Armed Services Committee, for an investigation into "the pattern of press leaks" on SALT hearings chaired in recent weeks by Jackson.

Culver's request for an investigation is seen by Senate aides as an indirect jab at Jackson and his staff. Culver has already endorsed the new SALT agreement.

Asked what he thought about the idea of an investigation into leaks yesterday. Jackson said he thought that might delay the more important business of consulting with executive branch on the new treaty during the final stages of its negotiation. he said, "I know of no leaks that have involved matters of national security."

Culver's letter referred specifically to two Evans and Novak columns that purported to describe events inside closed-door hearings in the Jackson subcommittee. The letter to Stennis was cosigned by Sens. Dale Bumpers (A-Ark.), Gary Hart (D-Colo.), Wendell R. Anderson (D-Minn.) and Thomas J. McIntyre (D-N.H.).

Jackson's subcommittee met yesterday to hear testimony on the SALT talks from Secretary of Defense Harold Brown.