A group of 18 Americans, including three House members, has canceled a fact-finding trip to South Africa after the Pretoria government refused to issue visas to all but four of them.

A South African Embassy spokesman said the visas had not been denied but said "the sensitive and delicate political circumstances prevailing in South Africa would make a visit of the nature contemplated by the Center for Development Policy, however well-intended, inopportune."

In a letter yesterday to the center, South African Ambassador Herbert Beukes said "the composition of the group further leads the government to believe that a visit at this stage would not be opportune," according to the center's executive director, Lindsay Mattison.

Included in the delegation were Reps. Mickey Leland (D-Tex.), head of the Black Caucus in the House, Douglas H. Bosco (D-Calif.) and George Miller (D-Calif.); Christopher J. Mathews, an aide to House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), and David Aarons, deputy national security affairs adviser in the Carter administration. Also in the group were two former ambassadors, Robert White and Walter Carrington.

The South African spokesman said visas were available for the three congressman and Mathews.

Leland, however, said no one would be going because the group was "not going to permit the extension of control by the South African government to the United States by selecting who can and cannot be part of a delegation."

The trip was scheduled to be a fact-finding trip, but a center official said the group also hoped to see leaders of all factions "to talk about an all-parties conference to begin negotiations of a settlement" of the racial conflict.

Meanwhile, a State Department official said the United States and European governments were pressing South Africa to reverse itself and allow the Rev. Allan Boesak, an antiapartheid leader, to travel here to receive the Robert F. Kennedy Human Rights Award Nov. 20.