President Carter has told three concerned U.S. senators that he will not permit a new strategic arms limitation treaty (SALT) to spur a new round of weapons competition, and asserted that the planned pact will "dramatically limit" the present avenues for a strategic arms race.

The president gave his views in a letter to Sens. Mark Hatfield (R-Ore.), George McGovern (D-S.D.) and William Proxmire (D-Wis.), who announced last month that indications of a new arms race make it "very difficult if not impossible" for them to support the SALT II accord.

In an effort to quiet fears and complaints from liberals, Carter also said that the decision about the production of a powerful new U.S. mobile missile, the MX, "must be made outside the context of SALT."

The Joint Chiefs of Staff as well as a number of conservation senators and outside political figures are reported to be insisting, as a price for supporting SALT, on approval of military programs such as the MX to deal with the future vulnerability of U.S. land-based nuclear missiles.

Carter told the three senators "we may find it desirable" to deploy an MX missile to replace the increasingly vulnerable silo-based missile force. He called this "a critically important and difficult decision." However, he pledged that "I will not be influenced by factors relating to SALT ratification in making my decisions on this or any other defense issue."

Proxmire, in announcing his concern about the SALT II environment in early March, had said the price of the treaty may be "too high" if the result to "make the world safe for the MX." He charged that SALT has become the vehicle for justifying the new missile program at a cost of $30 billion to $40 billion.

Carter's letter maintained that the impending treaty, which is in its final stage of negotiation with the Soviet Union, provides "much more restrictive limitations" in the quantity and quality of strategic offensive weapons than were agreed to in the SALT I pact of May 1972.

Acknowledging the failure of his initial effort to negotiate "deep cuts" in U.S. and Soviet weaponry, Carter said "I, too, would have preferred to see even more restrictive limitations in this agreement, but that did not prove feasible." He added that "we will never achieve those more restrictive limitations if we abandon this agreement simply because it did not do enough."

To buttress his claim that the treaty "dramatically limits" avenues for a new strategic arms race, Carter cited several SALT II provisions that have received little attention. They are prohibitions on the development, testing and deployment of certain new kinds of strategic systems including:

Ballistic missiles launched from surface ships. Such plans were considered in the Kennedy administration as part of the proposed "multilateral force" but never given final approval.

Missiles launched from fixed bases on the ocean floor. The 1972 Seabed Treaty prohibits placement of such weapons on the ocean floor but does not prohibit development or testing.

Fractional orbital systems, which could fire rockets from space against ground targets. Deployment of space weapons is prohibited by the 1962 Outer Space Treaty, but not development or testing of them.

In a related development, the State Department confirmed that $600,538 was spent last year by executive branch officials stumping the country to advocate a SALT pact. The figure was reported to the General Accounting Office and first made public by Sen. Barry Goldwater (R-Ariz.), a critic of the treaty.

Senior State, Defense and Arms Control Disarmament Agency officials were principal speakers on 11 conference on SLAT, the report said.