President Reagan will challenge the Soviets on human-rights issues and the war in Afghanistan during three days of speeches and meetings at the United Nations designed to regain the diplomatic initiative in advance of the November summit meeting in Geneva, administration officials said yesterday.

"We must address the most important fundamental differences between our two countries, which are the underlying reasons for our large military expenditure," White House spokesman Larry Speakes quoted Reagan as telling Republican congressional leaders in discussing his plans for a U.N. address. "Our agenda will include human rights, bilateral contacts and Soviet expansionism in the Third World."

Reagan's comments, which officials said will be reflected Thursday in his address to the U.N. General Assembly, represent a change of tone in the White House, where the focus has been on arms control and Reagan's Strategic Defense Initiative, often called "Star Wars."

"We want to expand the agenda," said one official. "We want to put the Soviets on the defensive."

Reagan's attempt to command the agenda at a U.N. meeting that will be attended by more than 80 heads of state comes after a month of Soviet diplomatic initiatives that U.S. officials regard as highly effective.

They include a Soviet proposal for reducing some categories of nuclear weapons by 50 percent, a U.N. speech by Soviet Foreign Minister Eduard Shevardnadze calling for "Star Peace" and a highly publicized trip to Paris by Soviet leader Mikhail Gorbachev.

The administration's attempt to shift the emphasis to issues that one official said "tend to make the Soviets uncomfortable" was reflected in a briefing by a senior administration official who usually emphasizes the importance of arms control. Yesterday, the official listed arms control last among the four agenda items that the United States wants discussed when Reagan meets Gorbachev at Geneva on Nov. 19-20.

When the official was asked whether nuclear-arms reductions would be Reagan's chief goal at the summit, he said that arms control was "terribly important" but added that "it's also important that we limit the use of force to expand influence." This is the diplomatic phrase frequently used by U.S. officials to discuss the Soviet war in Afghanistan.

Pakistani President Mohammed Zia ul-Haq said Monday in New York that the Soviets, because of heavy casualties, eventually will conclude that "there is no military solution" in Afghanistan. He said that Gorbachev is taking a "wait and see" attitude toward the war while dealing with internal issues.

A senior administration official yesterday praised Zia's refusal to bow to Soviet pressure on Afghanistan and his decision to meet with Indian Prime Minister Rajiv Gandhi at the United Nations. Reagan will hold separate meetings with both leaders during his three days in New York.

There also are tentative plans for the president to meet with Shevardnadze but not with Nicaraguan President Daniel Ortega, who also will attend the session.

Senate Majority Leader Robert C. Byrd (D-W.Va.), who discussed the Afghanistan issue with Gorbachev in Moscow last month, told reporters Monday that one outcome of the summit could be a mutual pledge to abide by the 1972 ABM treaty and a decision by Gorbachev to pull out of Afghanistan.

Byrd said that since he returned from Moscow a source whom he wouldn't identify had told him that Gorbachev could abandon Afghanistan since "it didn't happen on his watch" and that the Soviet leader "could make quite a splash" by announcing the pullout at the summit.

Whether the change in the administration's tone means a corresponding change in position at the Geneva summit or is merely tactical was not clear. One official said the U.N. meeting is "all public diplomacy" and intended to counter Gorbachev's visit to Paris, where he met with French President Francois Mitterrand.

Mitterrand is the one Western leader who did not accept Reagan's invitation to a "mini-summit" of the seven western industrialized democracies whose leaders meet each year at an economic summit. The leaders of the other nations -- Britain, Canada, West Germany, Italy and Japan -- have accepted invitations and are to meet privately with Reagan in New York.

Heavy attention will be focused on Reagan's meeting Thursday with Italian Prime Minister Bettino Craxi, whose government fell last week because of the political repercussions from the U.S. capture of the four hijackers of the Achille Lauro but has since been reconstituted. At the Bonn summit meeting this year the Reagan-Craxi private meeting was repeatedly postponed; this time, said an official, "it will be center stage."

Administration officials said the speech Reagan will give to the General Assembly will be blunter on human-rights issues and Afghanistan than the one he gave in the election year of 1984, when he emphasized arms control.

Sources said the original draft of this year's speech, written by campaign speechwriter Kenneth Khachigian in collaboration with senior White House speechwriter Ben Elliott and communications director Patrick J. Buchanan, had a harder edge than the final version approved by Reagan. The sources disagreed, however, on the extent of the modifications, though they all said the tone had been modified during a discussion with the president Friday.

In one change on the agenda that could have political significance, Vice President Bush was substituted for Reagan as the U.S. speaker at a commemorative ceremony Thursday afternoon in honor of the 40th anniversary of the founding of the United Nations.

A Republican source close to the administration said the Bush speech has been in the works for some time and is a deliberate attempt to build up the vice president, partially with an eye to domestic politics. The senior official who briefed reporters on the U.N. meeting emphasized Bush's experience at the United Nations, where he served as ambassador during the Nixon administration.