President Carter's well-intentioned plan to cut back U.S. arms sales abroad in hopes that other countries would do the same hasn't worked very well, partly because of the administration's own actions and partly because the Soviets and Europeans are selling more weapons than ever to nations of the Third World.

That was the assessment in a report released yesterday by the Senate Foreign Relations Committee on the president's policy, announced with considerable fanfare three years ago, of unilateral restraint in the sale of conventional weapons to countries other than traditional U.S. allies.

The idea behind the president's original plan was to curb the world-wide flow of arms and violence, with the unilateral U.S. example supposedly encouraging others to show restraint.

"From the outset," however, "the Carter arms transfer policy was beset with difficulties, in part because the policy had been oversold" by the administration, the Senate report says.

While administration spokesmen praised the concept of restraint, within months of the May 1977 policy proclamation the White House made a major exception to it by selling sophisticated AWACS early warning planes to Iran, the report noted.

Then it sold F15 fighters to Saudi Arabia, lifted the arms embargo on Turkey and provided $800 million in military aid to South Korea.

Even though a newly established Arms Export Control Board rejected many more potential arms deals, "the approval of such controversial sales tended to overshadow efforts at restraint and deflated the expectations of many in the arms control community," the report said.

"In the final analysis," the report says, the Carter policy has had a "modest impact" in terms of shifting the burden of proof from those who oppose a sale onto those who favor it. "But in general, arms sales have not proven to be the 'exceptional foreign policy instrument'" that President Carter called for in 1977.

The president meant that such sales should be made only in exceptional circumstances. But the committee says that is unrealistic. The United States should continue to be restrained and prudent in arms sales "while at the same time recognizing that in particular circumstances they may well serve important foreign policy and national security functions."

The Soviet invasion of Afghanistan has reemphasized the U.S. need for a variety of instruments to serve foreign policy, it said, including arms sales.

The Carter Administration itself has abandoned much of its own policy in recent months, setting a less restrictive ceiling this year on arms sales abroad, allowing U.S. firms to develop fighter planes specifically for export, offering new arms aid to Pakistan and to Oman, Kenya and Somalia in return for access to airfields and ports in those three countries.

In effect, committee sources said, the report can be seen as an attempt to make U.S. policy match actions. "There is no sense saying one thing and doing another," one source said.

The administration clearly is not implementing its own policy vigorously anymore, another source said. And, as the report points out, 1978 negotiations with the Soviets yielded no real progress, and newly released statistics show both Moscow and the major West European arms suppliers -- particularly the French and the British -- have significantly increased their sales in the past three years.

Those U.S. government statistics show that the Soviets in 1979 exceeded the United States for the first time in arms deliveries to the Third World, with about $8 billion in sales compared to $5.7 billion for the United States.

In terms of sales to the Third World, the 1977-1979 annual average for the four major West European suppliers was $7.2 billion combined, as compared with $3.4 billion for the three previous years, the report said.

The committee argued that the United States should have a "balanced" policy that follows a "middle course" and suggested several modifications to the president's three-year-old declaration in an effort to make that policy more realistic.

The committee recommended that the president should drop the idea of an annual ceiling on such sales because it is based" on the questionable assumption that the gross volume of sales itself, rather than judgement relating to each individual sale, is of significant importance."

It recommended that the White House take a second look at its policy of prohibiting development of weapons solely for export in light of its own decision to permit the FX fighter to be developed. The committee feels that in certain circumstances such exports can be beneficial for U.S. policy. t

It also urged the White House to review and modify its restrictions on co-production by other countries of certain U.S. weapons so as to align official policy "with current practice."