A simple change in semantics in the Carter administration's proposal to sell jet fighters to three Mideastern nations has significantly improved the prospect that Congress will allow the sales, Senate Majority Leader Robert C. Byrd (D.W. Va.) said yesterday.

Byrd said the administration's decision to drop "the semantic buzz-word 'all-or-nothing package' . . . should go a considerable distance to defuse the objections" in Congress to the sales. At the moment, he said, there is majority support in Congress for them.

Byrd's comments came at his regular Saturday press conference. On Friday, President Carter said Israeli Prime Minister Menachem Begin did not express concern about the sales when he visited here last month.

"For two days he had my undivided attention and he never mentioned it," Carter was quoted as saying in a White House transcript of his Friday press conference with visiting editors and news directors. The transcript was released yesterday.

Defending the sales Carter said, "I think it would be a serious mistake for us to sever the friendly relationships and the mutual trust and confidence that is crucial that does exist between ourselves and the moderate Arab leaders . . . I think this is a well-balanced proposal."

The administration formally notified Congress Friday of its intention to sell 60 F15s to Saudi Arabia, 50 F5Es to Egypt, and 75 F16s and 15 F15s to Israel. The F15 is the most advanced jet fighter in the U.S. arsenal.

The administration made no reference in the notification to its previous insistence that Congress consider the sales to the three countries as a package which would stand or fall as a single entity.

Asked if the delegation of the word "package" were anything more than a semantic charge, Byrd replied, "It was semantical from the beginning. There is no such thing as a package. It has no legal basis . . . To use that term created a lot of concern here."

Byrd said he supports the $4.8 billion transactions, but he predicted it will be no easy task to avoid a congressional vote to prohibit the sales. "It's still going to be difficult, but the prospects have improved," he said.

Congress has 30 days from last Friday or until May 28, to vote against the sales. In order to block them, resolutions of disapproval must be passed by a majority in each house.

Byrd noted that even though the administration did not refer to the sales as a package in notifying Congress, it "still retains its option" of withdrawing the proposal if Congress disapproves any of it.

Selling warplanes to Saudi Arabia has provoked the largest part of the opposition to the proposal. Byrd pointed out yesterday that, "If we don't sell these plane to Saudi Arabia, they will get them somewhere else - they have got the bankroll."

Carter said in his press conference that the Saudis plan to use the planes only in a defensive role. "My own belief is that the Saudis have made their choice of weapons and the appurtenances or armaments on the F15 on a basis of defense . . . The Saudis have not ordered the air-to-ground armaments that would be use in an offensive mode."

Both Carter and Byrd stressed that the United States will be able to exert more control over the use of weapons it sells than over weapons purchased from other arms suppliers.

Byrd said he does not think Carter has augmented his growing reputation for waffling by dropping the package reference.

"He has done a realistic and reasonable thing," Byrd said. "One can be strong, one can be tough, but one must be flexible when flexible is the right thing."