Three liberal senators -- Mark Hatfield (R-Ore.), George McGovern (D-S.D.) and William Proxmire (D-Wis.) -- announced yesterday that they now find it (very difficult, if not impossible." to support ratification of the strategic arms limitation treaty (SALT II) now being worked out with the SOVIETS.

Reminding President Carter that the margin of victory when the Senate votes on ratification "could be" three votes, the senators said they believed the treaty, coubled with administration plans to proceed with the development of several new weapons systems, may represent "a redirection of the arms race rather than" a curtailment.

The statement, in effect, formally notified Carter that he risks the loss of liberal support for the treaty should he go too far in reassuring conservatived senators.

The three indicated that Carter already may have stepped over the line in his Feb. 20 foreign policy speech at Georgia Tech when he said SALT II will allow the United States to "pursue all the defense programs we believe we may eventually need," including the MX missile.

In a letter to the president, the three senators said that "your recent argument that it [the treaty] constrains the momentum of Soviet programs while allowing the United States to build up does not give us confidence that the treaty embodies a true step toward arms reducations."

Hatfield called the president's posture "contradictory at best and deceptive at worst."

McGovern said "it appears as if the Carter administration is ready to sacrifice our long-term hopes for comprehensive arms control in order to win a few hardliners' votes for a very modest interim step in the SALT II agreement.

"The design of this treaty," he said, "is not to achieve the best arms control framework; rather, our diplomats have been told to negotiate around and protect the Pentagon's with list for a whole new crop of strategic weapons."

A two-thirds vote of the Senate will be required for approval of SALT II. Getting those votes is a major preoccupation of the Carter administration in this session of Congress.

Much of the White House's effort has been spent in trying to convince critics -- in the Pentagon as well as outside the government -- that the treaty will not handicap the United States in its stragegic relationship with the Soviets. The promise of the MX missile -- a new weapon designed to be moved from one location to another to prevent destruction -- has been a primary administration argument in dealing with these critics.

The MX is also at the heart of the Proxmire-Hatfield-McGovern warning. "The price of SALT II may be too high," Proxmire said. "One of the great ironies of the proposed agreement is that it will make the world safe for the MX. SALT will be the vehicle for justifying the MX program, which will cost at least $30 billion for the land-based version.

"What we need," Proxmire continued, "are real reductions in the land-based missiles on both sides."

Administration sources suggested that such pressure from the liberal flank may be useful to them by creating a counterforce to the heavy conservative lobbying underway against SALT II.

Ultimately, according to this view, liberals such as McGovern, Hatfield and Proxmire will vote in favor of ratification despite their specific objections.