Three kidnaped Britons were found shot to death near Beirut today, a British journalist was seized by gunmen and the British ambassador's vacated residence was shelled in a new outbreak of violence apparently in retaliation for Britain's support of the U.S. raid on Libya.

The attacks came two days after a group headed by pro-Libyan terrorist leader Abu Nidal declared here that "Palestinian, Arab and revolutionary forces" would "answer back harshly in pursuing and striking against" U.S. interests and "countries that have given in to pressures" from the United States.

A note found near the bodies of the three British captives, all reportedly killed by gunshot wounds at close range, threatened more violence in retaliation for this week's raid on Libya and said that "U.S. and British imperialism and their interests will be our targets."

While the British Embassy withheld official identification of the three slain men, friends, an Irish diplomat who viewed the bodies at the American University Hospital and doctors there said they were journalist Alec Collett, 63, and teachers Philip Padfield, 40, and Leigh Douglas, 34.

Collett, who worked for the U.N. Relief and Works Agency, had been kidnaped on March 25, 1985. Padfield, director of a language center, and Douglas, a political science professor at the American University of Beirut, had been seized together on a Beirut street last March 28.

Dr. Ahmed Harati, the Lebanese government coroner who examined the bodies, told The Associated Press that Collett and Douglas were shot once in the head and Padfield twice, all from less than a yard away.

Earlier today, the ornate white stone mansion that is the official residence of Britain's ambassador here was attacked by rocket-propelled grenades. Three grenades pierced the walls of the building in Moslem west Beirut near the Green Line dividing the city, but there were no casualties, British officials said. Ambassador John Gray, who inspected the damage, has been living elsewhere recently.

About two hours after the attack on the embassy residence, the acting bureau chief of WTN, the London-based Worldwide Television News, John Patrick McCarthy, 29, was grabbed by four gunmen on his way to Beirut International Airport for a flight to London. He was accompanied by Lebanese colleagues in a two-car convoy, but an automobile blocked the road and forced them to stop.

"Four gun-wielding men emerged from the car. Some of them commandeered McCarthy's automobile and drove off with him to an unknown destination," WTN Vice President Roby Burke said in London.

McCarthy had spent a month here, replacing a colleague on vacation.

A variety of groups and callers, most of them apparently pro-Libyan and some of them linked to Abu Nidal, asserted responsibility or roles in the day's attacks. Some were unknown and others were small, previously insignificant groups that flourished in the first few years of Lebanon's civil war with Libyan financing and now appear eager to court Tripoli again.

The Voice of Lebanon, a Christian-run radio station, said an anonymous caller speaking for the Omar Mukhtar squad asserted it had attacked the British Embassy residence and a caller for the Organization of Moslem Martyrs said his group abducted McCarthy. Omar Mukhtar was a Libyan who led the revolt against Italian rule in Libya. Neither group had been heard of before.

The radio quoted the Omar Mukhtar caller as saying the attack on the British ambassador's residence had been carried out "in retaliation for his government's support to the U.S. in the attack against Libya."

The hand-scribbled note found on the bodies of the three slain Britons was signed by the Arab Commando Cells, a name that had not been used before but that resembled others used by groups linked to Abu Nidal and Libya.

The Organization of Socialist Moslems, which had asserted responsibility a year ago for the abduction of Collett, announced late tonight that it had killed him and it called on other groups holding American and British captives to "carry out the verdict of death against them."

The site near Roweisat Sofar where a villager found the three bodies today is an isolated stretch of mountain road controlled by the Druze Progressive Socialist Party militia and accessible to Syrian troops and other Palestinian and Middle Eastern groups operating in Lebanon's Bekaa Valley.

The bodies were taken by Druze officials to a hospital in Bhamdun and then to the American University Hospital by Lebanese and International Red Cross officials.

The Druze community has been a traditional friend of the British from days before Lebanese independence. Although Walid Jumblatt's Progressive Socialist Party has tried to keep up cordial ties with Britain it is also known to receive money from Libyan leader Muammar Qaddafi to fund its fighting against Lebanon's Christians.

News of the U.S. air raid Tuesday prompted many Britons and westerners who had hesitated to leave chaotic Lebanon despite the recent surge in kidnapings to hurriedly pack up and leave.

Today, a handful of western journalists who were still holding out in west Beirut -- among them Canadians, New Zealanders and an American photographer -- were flown to Jounieh, a Christian port city north of Beirut, in a military helicopter.

A number of teachers hoping to complete their terms at the American University and other institutions are now confined to their homes and the campus compound.