The United States and Soviet Union are planning to announce agreement today to begin their new three-way negotiations on nuclear and space arms in March, administration sources said last night.

The prospects for success in the negotiations, which are expected to take place in Geneva, "are somewhat better than in the past but I really couldn't say they are very good," said Ambassador Paul H. Nitze, special adviser for the talks and a central figure in the preparations.

As the negotiations draw nearer, Nitze said, the two sides were far apart on a new U.S. "strategic concept," which incorporates President Reagan's Strategic Defense Initiative (SDI), or "Star Wars" plan.

Secretary of State George P. Shultz and Soviet Foreign Minister Andrei A. Gromyko hammered out an agreement on the subjects and objectives of the negotiations in their meetings Jan. 7-8 in Geneva. At that point, however, the Soviets did not agree to a U.S. proposal to begin the negotiations in early March in Geneva.

The two sides pledged to work out the time and site through diplomatic channels within a month, and were able to do so in 17 days, about half the alloted time.

In the meantime, the United States has chosen and announced the three leaders of its delegation: Washington attorney Max M. Kampelman, former senator John G. Tower (R-Tex.) and career diplomat Maynard W. Glitman.

The ruling Politburo of the Soviet Communist Party approved the leaders of its delegation in a meeting Thursday and is expected to announce them in a news conference scheduled by the Soviet Foreign Ministry for 7 p.m. today Moscow time (11 a.m. Washington time).

The White House has prepared for the same time an announcement about the opening of the talks, officials said.

Nitze, in a breakfast meeting with reporters, said that before the Shultz-Gromyko meeting, Reagan had adopted a new "strategic concept" for the next 10 years that will be the centerpiece of the U.S. position in the coming talks.

The new strategic concept was drafted by Nitze in mid-December, approved by Reagan in his Jan. 1 instructions for Shultz, presented by Shultz to Gromyko in their talks and made public in a Jan. 14 speech in New York by Deputy Secretary of State Kenneth W. Dam. However, Dam did not reveal at the time that the "strategic concept" he summarized in the speech was a word-for-word restatement of the U.S. negotiating position, and scarcely anyone noticed his remarks.

The first part of the U.S. position, as made public by Dam and repeated yesterday by Nitze, is that "for the next 10 years we should seek a radical reduction in the number and power of existing and planned offensive and defensive nuclear arms, whether land-based, space-based or otherwise."

Both the United States and Soviet Union have pledged to seek such reductions in offensive nuclear weapons. Reagan's Star Wars plan, defined as a "non-nuclear" research program, evidently would not be covered in these cuts.

The second element of the U.S. position is that "We should even now be looking forward to a period of transition, beginning possibly 10 years from now, to effective non-nuclear defensive forces, including defenses against offensive nuclear arms." This assumes the success of the Star Wars research in the next decade and a transition to its deployment starting about 1995.

The Soviet Union, however, fundamentally opposes such a transition to a new sort of anti-missile defense based in space, considering it part of an "offensive" plan to tip the balance of effective nuclear might against Moscow.

Gromyko said in a Jan. 13 news conference in Moscow that "we will fight to the end" against "deployment of arms in space" and "the spread of the arms race to space," which are Soviet descriptions of Star Wars.

The Soviets are seeking to head off the "transition period" to space-defense systems by insisting on a total ban on construction, testing and deployment of space-based attack weapons, including defensive systems. A third element of the "strategic concept," as made public by Dam and Nitze, is that "this period of transition," which is to begin about 1995, "should lead to the eventual elimination of nuclear arms, both offensive and defensive. A nuclear-free world is an ultimate objective to which we, the Soviet Union and all other nations can agree."

Shultz and Gromyko agreed in Geneva on the overall objective of "the complete elimination of nuclear arms everywhere," but the Soviets rejected the anti-missile defense as a way to help accomplish this.

Nitze told reporters that, in the space portion of the three-way negotiations, the United States intends to discuss its concerns about "erosion" of the 1972 Anti-Ballistic Missile (ABM) Treaty. Among the problems on the U.S. list, he said, are a massive phased-array radar being erected by the Soviets in Siberia and Moscow's blurring of distinctions between fixed and mobile systems, and between antiaircraft and anti-missile systems.

New U.S. positions are being prepared for the strategic arms and intermediate-range arms portions of the negotiations, Nitze said, expressing hope that the Soviets are doing the same.