Three American clergymen arrived here today at the invitation of militants occupying the U.S. Embassy to hold Easter services for the estimated 50 hostages inside.

A spokesman for the militants said the clergy would conduct services for the American hostages "probably in the afternoon" Sunday and would be allowed to meet individually with them.

It was not immediately clear, however, whether the ministers would be allowed to see all the captives, a subject of U.S. concern during past visits by outsiders to the occupied embassy.

The three U.S. clergymen -- the Rev. Jack Bremer of Lawrence, Kan. and the Rev. Nelson Thompson of Kansas City, Mo., both Methodists, and the Rev. Darrell Rupiper of Omaha, a Roman Catholic -- issued a statement expressing opposition to the holding of hostages but sympathy for the Iranian militants' cause.

Their trip was made on behalf of "The Committee for American-Iranian Crisis Resolution," based in Lawrence and claiming chapters in 25 states.

The militants' preparations to allow Americans to visit the hostages for the first time since Christmas came as Iran's hard-line Moslem clerical party significantly toughened its stand on the controversial issue of transferring the Americans to government control.

There also were indications that the Islamic Republican Party's first outright declaration today of strong opposition to a hostage transfer was supported by revolutionary leader Ayatollah Ruhollah Khomeini. The Islamic leader so far has declined publicly to express a personal opinion on the issue, but he is reliably said to oppose a transfer.

Meanwhile, President Abol Hassan Bani-Sadr, one of the main proponents of a transfer, called on the Egyptian people to overthrow President Anwar Sadat and return the deposed shah to Iran.

The appeal to "the interests of the Islamic nation" appeared calculated to encourage the demonstrations against the former shah and Sadat that have become increasingly troublesome for the Egyptian government.

"The demonstrations of the Egyptian students and youths indicate that the Egyptian nation is not in favor of the Egyptian dictator," Bani-Sadr said in a statement broadcast by official Iranian radio. "The Iranian nation wants from you, the Moslem and civilized nation, to rise up against the puppet regime and wants the deposed shah to be returned to Iran."

He added, without elaboration, that Iran "is always prepared to crusade for the independence and freedom of all Islamic lands."

While the statement contributed to Bani-Sadr's revolutionary credentials, there were other signs that his hardline clerical adversaries, who form the core of the Islamic Republican Party, were continuing to chip away at the eroding power of the Iranian president.

In a statement in its party newspaper, the Islamic Republican Party said, "The hostages must be in the hands of the Moslem students following the imam's [Khomeini's] line as befor so that, as the imam has ordered, the Islamic parliament, which will be the highest and most powerful legal decision-making body of the country, can declare its decision in this respect. And we have no doubt that we must never surrender to U.S. threats, which are all empty."

The paper denounced the proposed hostage transfer as "very advantageous to the U.S."

The characterization of the parliament as Iran's supreme decision-making body came as a swipe at Bani-sadr's presidency and reflected a power struggle between him and the Islamic Republicans. The party is expected to control parliament, which may convene next month after a runoff election.

The embassy militants who have been resisting Bani-sadr's efforts to secure the transfer said they would allow hostages to go out into the embassy compound on Easter and would seerve them "something special to eat."

Invited by the militants to conduct Easter services along with the U.S. clergy were Greek Catholic Archbishop Hilarion Capucci and two Iranian Christian priests. Capucci arrived late yesterday for his third visit to Iran since the February 1979 revolution.

On their arrival in Tehran today, the U.S. clergy stressed that their visit was strictly a "pastoral" one and that they were not carrying any U.S. message to the militants or Iranian leaders or letters to the hostages. u

Bremer, the leader of the group, expressed thanks to "our Iranian brothers" for the invitation and lamented a "lack of understanding" between the United States and Iran.

Rupiper said the group was invited "because the students are religious people."

The willingness of the militants to allow the group's visit also apparently reflected the fact that it was organized by the captors. The militants last month succeeded in scuttling a planned visit with all the hostages by a U.N. commission, which the captors viewed as part of a U.S.-engineered plot to bring about the Americans' release.

In a statemend drafted before leaving Kansas for Iran, the clergymen said:

"The committee holds the taking of hostages as totally unacceptable but views this illegal act in the context of the public record of 25 years of U.S. government illegal intervention in Iran, which included the overthrow of a democratic republic, the imposition and fostering of a military dictatorship, the abuse of an economy and culture and complicity in policies and events which led to the torture, imprisonment and deaths of hundreds of thousands in violation of the American people's historic commitment to international justice and human rights."

The statement called for a congressional investigation into the past quarter century of U.S.-Iranian relations, U.S. acknowledgement of "the rights of the Iranian people to place the shah on trial" and Washington's cooperation in the investigation of the deposed shah's alleged crimes and in the return of any misappropriated funds.