Militant Moslem students holding 50 of the 53 American hostages in Iran said today that some of their captives have been moved from three towns to unspecified locations.

Three Americans have been held at the Iranian Foreign Ministry.

The captors, who have claimed they dispersed the hostages to 15 towns across Iran after an unsuccessful U.S. rescue mission in April, said the new moves were a continuation of their policy to relocate the captives. But their statement hinted that the transfers may have been prompted by attempts to kill some hostages.

"In view of military aggression by the world-devouring U. S. A. against Iran and its satanic plots against the lives to transfer them to different parts of the country so as to have them under the control of the nation, until such time as their final fate is decided" by parliament, said the statement, which was broadcast on the state-run radio.

"We announce that we shall continue our policy of transfers," it said.

"Consequently, we have transferred hostages from the three towns of Arak, Mahallat and Najafabad to other sites." Arak and Mahallat are southwest of Qom, and Najafabad is west of the central city of Isfahan.

The militants did not indicate how many hostages had been moved or where they were taken.

Meanwhile, two Britons, today were given 48 hours to leave Iran after being held incommunicado by unidentified authorities. Journalist Christina Powell and Persian expert Roger Cooper, both 45, were arrested while having dinner at Powell's home last night and held for about 17 hours in an unmarked building in central Tehran. Both had lived in Iran for many years.

President Abol Hassan Bani-Sadr asked Iran's chief prosecutor to close the Agence France-Presse bureau and expel its bureau chief, Jean-Jacques Cazaux, according to Tehran radio broadcasts.

They said Bani-Sadr accused the French news agency of distorting its account of an interview he gave to the French newspaper Le Monde last week. Bani-Sadr had spoken of his government's achievements on the economic front and in reestablishing discipline in provinces where rebellious Kurds have been fighting for autonomy.

In other developments:

Iran's roving Islamic Judge, Ayatollah Sadegh Khalkhali, was injured in a car accident last night, the official radio reported. It said Khalkkahli, who has ordered the executions of about 170 persons since launching an antinarcotics drive May 3, was in fair condition in a Tehran hospital after suffering minor chest injuries in the accident. No other details were given. In a newspaper interview conducted shortly before the accident, Khalkhali was quoted as saying that the Mafia was out to kill him for his antidrug crusade, but that he would get the Mafia first.

An Iran Air domestic flight arrived in Tehran five hours late yesterday because of a row over the stewardesses' clothes, Tehran newspapers reported today. They said two Revolutionary Guards on the plane objected that the women were not wearing Islamic dress, as ordered recently for government employes.