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LONDON, May 5- Britain ordered the expulsion of three South African diplomats today in retaliation for Pretoria's reported involvement in an arms deal with a paramilitary group in Northern Ireland.
Today's expulsion order is Britain's diplomatic response to an arrest made in Paris on April 21, breaking up a plan in which South Africa would sell arms to Protestant militants from Northern Ireland in exchange for British-made missile parts and designs. Five people were arrested in the joint British-French intelligence operation: three Protestant paramilitary figures, a South African diplomat and an American arms dealer based in Geneva.
The arrest of the South African diplomat has been deeply embarrassing to the Pretoria Government, prompting a personal apology from President P. W. Botha to Prime Minister Margaret Thatcher, regretting the ''transgression.'' But the deal also appears to indicate the lengths to which South African officials will go to try to sidestep a United Nations arms embargo and secure advanced weapons technology.
Since the arrest of the diplomat, Daniel Storm, the South African Government has sought to portray his efforts to obtain parts or a model of a British Blowpipe missile - a portable anti-aircraft weapon that roughly resembles a bazooka - as an unauthorized operation. Moreover, Mrs. Thatcher is often regarded as more sympathetic to Pretoria than other Western leaders because she has long resisted economic sanctions against South Africa.
Decision for Firm Action
Yet after consideration, the British Government decided firm action was warranted, given the involvement of a foreign government supplying wares to a terrorist group operating in Britain.
And the Prime Minister, who in 1985 narrowly escaped a bomb planted by the Irish Republican Army, pressed for sending ''a clear and unequivocal message'' to Pretoria, a British official said.
''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,'' a statement from the Foreign Office said.
The expulsion order was presented to the South African Ambassador, Rae Killen, at a 15-minute meeting with Sir Patrick Wright, the Permanent Under Secretary of the Foreign Office. The three South African Embassy staff members were not suspected of being involved in the arms-dealing plot. Sir Patrick also told Mr. Killen that Britain would view any tit-for-tat expulsions of British Embassy staff members in South Africa as ''entirely unjustified.''
The Blowpipe missile is produced at the Short Brothers aircraft and weapons factory in Belfast, which has suffered a series of thefts in recent months. The item seized in Paris was apparently a display model of the Blowpipe, not a working missile, according to police officials in Northern Ireland. The Blowpipe is a reliable but somewhat dated anti-aircraft missile that has been used since 1976.
The South African operation was apparently aimed at establishing contacts with people with access to Shorts' weapons secrets, and its real aim was to obtain information about the Shorts Starstreak missile, a high-velocity weapon still under development. In return, the Protestant paramilitary men were to receive arms and cash.
