LONDON (AP) _ Britain expelled three South African diplomats today because of their country's reputed role in a plot to sell arms to Northern Ireland extremists in exchange for British-made missile parts.

The Foreign Office said Undersecretary Patrick Wright summoned South African Ambassador Rae Killen and told him the three must leave the country within a week.

''We noted their expression of regret,'' a statement quoted Wright as telling Killen. ''However, the South African government should be under no illusions about the grave concern with which Her Majesty's government viewed the involvement by South African officials in this affair.

''Because of the gravity of this affair, Her Majesty's government has decided that the South African government should be requested to withdraw three members of the South African Embassy,'' the statement said.

The move came two weeks after authorities in Paris arrested a South African diplomat, an American arms dealer and three members of an extreme Protestant organization in Northern Ireland.

The Northern Irelanders allegedly were handing over parts of a Blowpipe, a sophisticated shoulder-fired, anti-aircraft missile manufactured in Belfast.

The British government expressed grave concern over allegations the South Africans were supplying weapons to the Protestants in exchange for the missile parts.

The South African, identified by French sources as Daniel Storm, was released after he claimed diplomatic immunity. A week later, the French government expelled three South African officials implicated in the affair.

The Foreign Office spokesman said the three who were ''requested to withdraw'' are First Secretary for Information Jan H.P. Castelyn, Counselor Jonathan Fourie, and Staff Sgt. Mark Brunner, who works for the defense attache's office. The statement contained no hint that they were involved in the alleged plot.

During the 15-minute meeting, Wright said, ''we had considered carefully the South African response,'' particularly a statement by Gen. Magnus Malan, South Africa's defense minister, to his country's Parliment on May 3.

At that time, Malan said an official inquiry into the affair had shown that Storm had acted without government approval. He said he regretted the embarrassment caused to the British, French and Irish governments and would act to ensure it would not happen again.

But he also acknowledged on April 24 that South Africa sometimes uses ''unconventional methods'' to get around an international arms embargo against his country.

The Irish government, which has no diplomatic relations with Pretoria, delivered a formal protest to South Africa.

According to the Foreign Office statement, Wright noted that South Africa is under a United Nations arms embargo and pointed to ''the security situation in Northern Ireland where the lives of innocent people are at risk.''

He also warned the South Africans that ''any retaliation would be entirely unjustified.''

The Roman Catholic Irish Republican Army is fighting to push the British out of Protestant-dominated Northern Ireland, join it with the Republic of Ireland and set up an all-Ireland socialist state. Protestant radicals are fighting the IRA effort.

The three Irishmen and one American, Douglas Bernhart, arrested with Storm were being held on gun-running and conspiracy charges.

On April 25, federal prosecutors in Bern, Switzerland, opened an inquiry into possible Swiss connections into the affair, saying Bernhart had an office in Geneva.

In Belfast, security sources said one of the three arrested was a warrants officer in the Territorial Army, a volunteer reserve force. As a non- commissioned officer, Samuel Quinn, 42, was thought to have instructed Territorial Army officers in the use of the Blowpipe.

The other Protestants were not named.