Britain today ordered the expulsion of three Syrian diplomats after their government turned down a British request that diplomatic immunity be waived to allow police to question them in connection with last month's attempted bombing of an El Al jet at London's Heathrow Airport.

A Foreign Office statement said the three, all listed as unspecified "attaches" at the Syrian Embassy here, must leave the country within seven days. It said that the British government "regretted" the Syrian refusal that had necessitated the expulsions.

The expulsions mark the first direct move against Syria amid a series of allegations of involvement in terrorist acts in Europe, including two recent bombings in West Berlin.

Officials here said that Damascus, which has denied any involvement in the El Al or Berlin incidents, had offered to allow the diplomats to be questioned under immunity inside the embassy, but police had declined on the basis that information from such diplomatically protected interviews could not be used in court.

The expulsions announced today follow agreements among the seven industrialized democracies at last week's Tokyo summit conference, and among the 12 members of the European Community, to take strong action against any state involved in sponsoring or supporting international terrorism.

In recent statements, President Reagan has said that he would use military force against Syria or Iran if presented with clear evidence that they had sponsored acts of terrorism against Americans. Alleged intelligence proof of Libyan involvement in the bombing last month of a discotheque in West Berlin, in which two persons were killed, one of them American, led to the U.S. air strike against Libya last month.

U.S. allies in Western Europe, while supporting the use of diplomatic and other sanctions in such circumstances, have been more wary of the efficacy of military action. Although British Prime Minister Margaret Thatcher approved and aided the U.S. strike against Libya, officials here indicated that today's expulsions could be interpreted as firm diplomatic action that would preempt the need for military retaliation.

In an interview today with the British Press Association, Syrian Ambassador Loutof Haydar said he had reported the expulsions to Damascus and expected to make a statement soon, possibly within 48 hours.

Haydar said that Syria's willingness to allow the three diplomats to be interviewed at the embassy was "an expression of our good intentions . . . and of confidence in the innocence of the Syrian Embassy and its staff."

"This, however, did not meet with the requirements of British justice," he said, "because the evidence that was taken could not be produced in court. But there has been no accusation or charge of any misdoing by anyone in the embassy. In these circumstances, we could not accept the request to waive diplomatic immunity."

Haydar repeated his earlier statement that "Syria has never had any knowledge or involvement in any terrorist activity on British soil."

It remained unclear whether Britain's desire to question them was intended to lead to the prosecution of the three diplomats, named by the Foreign Office here as Zaki Oud, Ahmad Abdul Latif and Mounir Mouna, or to assist in the prosecution of Nezar Hindawi.

Hindawi, 31, a Jordanian national, has been charged with conspiracy to commit murder in the April 17 bombing attempt. He allegedly gave his unsuspecting girlfriend a suitcase containing a concealed bomb as she was to board the El Al Israeli airlines flight to Tel Aviv from London's Heathrow Airport.

Israeli guards discovered the bomb during a security check at the departure gate. Questioning of the girlfriend led to Hindawi's arrest the next day, and the discovery that he had entered this country illegally on a false Syrian passport. Further investigation, along with the interrogation of Hindawi, led to the three Syrian diplomats.

Hindawi reportedly is the brother of Ahmed Nawaf Mansour Hasi, one of two persons being held in West Berlin for the April 5 discotheque bombing. West German police have said that Hasi, while denying participation in the disco case, has confessed that he and his brother planned the March 29 bombing of the German-Arab Friendship Society in West Berlin.

Police said that Hasi told them the explosives for the friendship society bombing had come from the Syrian Embassy in East Berlin. The Syrians have denied any involvement in the incident.

Although British officials have remained tight-lipped over the extent of their information implicating the Syrians in the El Al bombing case, Israeli officials repeatedly have said they have firm evidence of Syrian "collaboration." Israeli diplomats here, however, have acknowledged that their proof might not be admissible in court.

Israeli Police Minister Chaim Bar-Lev indicated Saturday that Israel might retaliate against Syria if there was clear evidence it was behind the attempted El Al bombing, Reuter reported. He told Israeli state radio that "no one is immune to such actions."

Asked if Israel would act against Syria if it became clear it was involved in the bomb plot, Bar-Lev said: "In principle, definitely yes." Prime Minister Shimon Peres said Friday that Israel did not intend to attack Syria.

The first public indication here of suspected Syrian government involvement came May 1, when Haydar was called to the Foreign Office. Antony Acland, head of the British diplomatic service, "raised with Haydar allegations about Syrian involvement in certain terrorist activities in this country," a Foreign Office statement released today said.

"On behalf of the police," the statement said, Acland asked Haydar to "waive the diplomatic immunity of three members of his staff to enable police to ask them questions about these allegations."

On Monday, after consulting with Damascus over the weekend, the British statement said, Haydar reported back to the Foreign Office that "his government was not willing to allow diplomatic immunity to be waived but would permit interviews with members of his staff on embassy premises."

Police concluded, however, "that interviews under such conditions in circumstances which could not result in evidence which might be used in court could not assist their investigation," the statement said.

This morning, Haydar was called back to the Foreign Office and told that the three diplomats were being expelled.