American ambassadors to three different arms-control negotiations with the Soviet Union met with President Reagan yesterday before returning to the bargaining tables. But they gave reporters no indication later that a breakthrough was near in any of the talks.

Ambassador Edward Rowny, who heads the U.S. delegation to the Strategic Arms Reduction Talks (START), said, however, that Reagan seemed encouraged that Moscow was moving "rather briskly" in these discussions on limiting intercontinental-range missiles and bombers.

Rowny said the pace of the talks, which began in Geneva last June, was "much more rapid" than the tempo of discussions in the early 1970s which lead to the now expired U.S.-Soviet strategic arms limitation pact.

Although Rowny declined to discuss details of the Soviet position in these talks, it is known from reliable sources that Moscow has proposed a ceiling of 1,800 missiles and bombers in each nation's arsenal.

This would represent about a 25 percent cutback in Moscow's existing array of weapons and about a 10 percent cut in this country's forces.

While these reductions would fall far short of the 850-missile ceiling proposed by President Reagan, the Soviet plan involves more significant cuts than Moscow has considered in the past and has attracted interest among a few officials here as a step in the right direction. The Soviets have also refrained from totally rejecting the U.S. plan.

The Soviet proposal, however, is certain to be rejected as it now stands, sources here say, because it does not go far enough and has several important catches.

One is that it ties intercontinental-range missile cutbacks to an American requirement not to deploy new intermediate-range cruise and Pershing II missiles in Europe.

Another is that the Soviets have proposed limiting only the number of launchers -- the silos where missiles are based -- rather than the number of missiles. This means that many extra missiles could be built and stored elsewhere and that it would be hard to limit the number of warheads on each missile.

The Soviets reportedly have "dangled" in front of U.S. negotiators some overall ceilings on missile warheads and the number of atomic bombs and missiles to be carried on bomber aircraft, but it is said to be unclear at this point what Moscow has in mind.

Under questioning, Rowny did reveal that a series of so-called "confidence-building measures" to reduce the risk of atomic war that were announced by Reagan with considerable fanfare in West Berlin last June have not yet been proposed to the Soviets at the Geneva talks. Rowny said he was hoping for further instructions on introducing these proposals within the next several weeks.

In a Berlin speech on June 11, Reagan had said "we shortly will approach the Soviet Union with proposals in such areas as notification of strategic exercises, of missile launches and expanded exchange of strategic forces data," all steps meant to reduce uncertainty about what each side was up to in nuclear matters.

Aside from the START discussions, which resume in Geneva on Oct. 6, the two superpowers will resume talks there Sept. 30 on limiting Intermediate-range Nuclear Forces (INF), which involve missiles and aircraft based in Europe. On Sept. 23 in Vienna, the eight-year-old Mutual and Balanced Force Reduction (MBFR) talks between the United States and its NATO allies and the Soviet-led Warsaw Pact will also begin a new round.

In a public statement accompanying the private White House meeting with Rowny, INF negotiator Paul Nitze and MBFR delegate chief Richard Starr, Reagan said "we are encouraged by the serious and businesslike conduct of these negotiations thus far" and "I am determined to bargain in good faith until our objectives can be realized."