American officials say the three suspects being held in London in connection with the shooting last week of the Israeli ambassador to Britain are not members of the Palestine Liberation Organization but belong to a dissident Arab terrorist group named "Black June" that now operates out of Syria and is openly hostile to PLO leader Yasser Arafat.

The assassination attempt on Ambassador Shlomo Argov last Thursday was a critical event in the sequence of actions leading up to Israel's invasion of southern Lebanon in an effort to crush PLO guerrillas there.

The day after the shooting of Argov, the Israeli military command launched heavy bombing attacks against Palestinian bases in the area of the Lebanese capital of Beirut and announced that these raids were in retaliation for the shooting. The Israelis said the assassination attempt was a breach of the cease-fire between Israel and the Palestinian guerrillas.

After the Israeli air raids on Friday, the Palestinians began an intensive artillery and rocket bombardment of northern Israeli towns. On Sunday, Israeli troops invaded southern Lebanon in retaliation for the shelling and for what Israeli officials called "constant acts of terrorism" carried out over many months.

Although the PLO immediately denied any responsibility for the assassination attempt, Israeli officials argue that it is the PLO that is ultimately behind all such terrorist acts wherever they occur.

The Israeli ambassador to the United States, Moshe Arens, told reporters on Monday that "we know enough about the PLO, its tactics and methods, to feel assured this was an act of the PLO, even though they won't take the credit. It is a well-known PLO tactic to come up with new names . . . but they are all one of the same Mafia-type octopus that works out of Lebanon."

Yesterday, another Israeli emissary said that the invasion was not a direct result of the assassination attempt. Rather, he called that attempt "only the straw that broke the camel's back," and noted that it had been preceded by a long list of alleged violations of the cease-fire across the Lebanese border that had been negotiated last July.

Nevertheless, according to U.S. sources, the apparent certainty that the would-be assassins are not members of the PLO, but are part of a group that split from it several years ago and is now hostile to it, is known to be causing some concern and dissension within the Reagan administration.

Some White House officials, for example, are known to have argued that the United States should have taken a tougher line against Israel's massive invasion and cited the fact that the assassination attempt was not even carried out by the PLO.

These officials are understood to have argued that the United States had asked Israel privately not to invade Lebanon unless there was a massive provocation and that the timing of the attack was an embarrassment to President Reagan, who was in the midst of his European trip and was also about to send a special negotiator to the Middle East to try to preserve the cease-fire.

According to sources here, the "Black June" group broke away from the PLO around 1975 and initially was supported by Iraq and operated out of Baghdad.

As relations with Iraq cooled, the group is said to have moved to Syria in 1980. The group is an extremist organization, linked with previous assassination attempts, and espouses the view that PLO leader Arafat is too soft on Israel. The group was initially headed by a Palestinian but has been vigorously opposed by the PLO and the mainstream of the Palestinian movement.

The suspects being held in London are said to be of "mixed" nationalities, but sources here say there is "no doubt" that they are members of "Black June." Two are reportedly Jordanian-born and the third was reported to be an Iraqi merchant.

The likelihood that the would-be assassins are not PLO members, as sources here seem to believe, also appears to have raised questions in London. Last Sunday, after the Israeli invasion began, British Prime Minister Margaret Thatcher reported that a "hit list" containing the name of a PLO official in London, who presumably was to be killed, was found on one of the suspects in Argov's shooting.

After Thatcher's statement on Sunday, an Israeli spokesman in London said it still was not known who was responsible for the shooting. American officials say they don't know when Israel learned of the would-be assassins' associations.

But the Israeli spokesman also said last Sunday that "we do not accept the idea that this attack was carried out by a breakaway group of the Palestine Liberation Organization. There is no such thing as a good and bad PLO."