A Sunday newspaper here has given an account of how the South Africans obtained photographs which they say prove three American diplomats were spying on South Africa.

Meanwhile, public opinion appears to support the government's expulsion of three offending diplomats last week, condemning the American government for not apologizing and for asking two South African military attaches to leave Washington.

The Sunday Express, which has criticized the government in past, commented today that the government had "acted perfectly properly in expelling the people concenrned."

Local papers have been told Prime Minister Pieter Botha will bring up the matter again in parliament this week, possibly revealing more about alleged U.S. intelligence gathering in South Africa.

Foreign Minister Pik Botha told one South African reporter: "Make no mistake, this is a serious issue and it won't just simply be forgotten." All signs point to increased hostility from Pretoria and a further deterioration in relations between the two countries.

Quoting "informed sources" who appeared to be South African, the Johannesburg Sunday Times today said that South African intelligence agents, after keeping a U.S. Embassy Beechcraft 200 under surveilance for six months, stole into the aircraft the night of April 4 as the three man crew slept in a hotel in Upington.

The South Africans took film from the camera under the copilot's seat. They developed it and found photos of "strategic regions" of South Africa, including Pelindaba, the site of this country's uranium enrichment plant and headquarters of its Atomic Energy Board, the paper said.

The government withdrew the right of the embassy to keep its won plane in the country last week and yesterday it was flown to neighboring Botswana under the vigilant escot of two South African Air Force planes.

Upington is in the sparsely populated northwest corner of South Africa. In 1977 the U.S. government said that South African was setting up a nuclear weapons testing site there. South Africa denied it.

Many South Africans one talks to believe the Americans were attempting to get information about the existence or nonexistence of this site and about the country's nuclear weapons capability. The South Africans have been particulary cagey on this subject with Americans, who have been pressuring Pretoria to sign the Nuclear Nonproliferation Treaty.

The argument about espionage almost certainly ensures that the chances South Africa will sign that treaty have moved from probable, as the Carter administration was predicting last year, to highly unlikely.