The Soviet Union appears to be working on two new long-range bombers and modifying an existing one for possible deployment in the mid-1980s, Pentagon officials said yesterday.

Pentagon research executives have been predicting that the Kremlin would go beyond the present-day Backfire in developing bombers. Whether the Soviets will end up building all three of the bombers, or abandon them before entering production, is a question that U.S. intelligence officials cannot answer, sources said.

The Soviets traditionally develop several prototypes of one weapon before choosing one for quantity production or canceling all of them.

Intelligence officials supsect that the Soviets are designing two intercontinental range bombers, with at least one of them like the U.S. Air Force B1 President Carter conceled.

There is no consensus on the design of the second bomber, sources said, while the third is believed to be an improved version of the Soviet Backfire or Bear bombers already flying.

If the evidence hardens that the Soviets intend to build and deploy a long-range bomber around 1985, Pentagon sources said, the most logical American response would be to develop a new bomber rather than invest in air defense.

The Air Force already is exploring advanced bombers to succeed the B1. The effort could be stepped up in Fiscal 1981, sources said, but no crash effort is envisioned.

Associated Press reported yesterday that a Soviet negotiator at the U.S.-Soviet summit in Vienna earlier this month surprised American officials there by disclosing that his country was working on three new heavy bombers.

A Pentagon executive said yesterday the negotiators "shouldn't have been" surprised that the Soviets had more than one bomber under development. He said sketchy intelligence information on the two new bombers and modified one has been in hand for some time.

The Backfire bomber is not counted as an intercontinental bomber under the pending strategic arms limitations treaty and thus is not restricted within it. However, the Soviets have promised to limit Backfire production to the present rate of 30 a year.

A new-range Soviet bomber would be counted under SALT II, just as are the U.S. B52, bombers.