NELSON Mandela has come under some criticism for softness on Libya's Moammar Gadhafi, the PLO's Yasser Arafat and Cuba's Fidel Castro, three Third World supporters of the African National Congress who are rightly the objects of severe criticism in the United States. It is an important matter for Mr. Mandela, who is eager to solidify American support on his current visit, and in television and press interviews he has been addressing the question in what we find to be generally a careful way.

He has said, unexceptionably, that it was mistaken for others to think merely that their enemies should be the ANC's enemies. The ANC's own attitude to Col. Gadhafi and company, he said, was based ''solely on the fact that they fully support the anti-apartheid struggle.'' The ANC turned to Cuba, in fact, only after being rebuffed by the United States. Mr. Mandela hailed the commitment of the three controversial leaders to human rights -- pay attention here -- ''as they {rights} are being demanded in South Africa.'' The ANC, he went on, has ''no time to be looking into the internal affairs of other countries.''

On the PLO, he offered an explanation that a fair number of Israel's American friends might grant -- that the ANC identifies with this group ''because, just like ourselves, they are fighting for the right of self-determination.'' He added that the ANC has stood ''quite openly and firmly for the right of {Israel} to exist within secure borders'' -- minus the ''conquered'' (more fairly, occupied) territories. ''Until February this year, when he had to accept us as we are,'' Mr. Mandela disclosed in an aside, Col. Gadhafi had kept the ANC from opening an office in Libya because the ANC had told him it ''works with Jews in our organization.''

Pressed further by ABC's Ted Koppel, Mr. Mandela expressed sympathy for Jewish travails and admiration for ''the lack of racialism amongst the Jewish communities,'' acknowledged the Jewish lawyers who had defended the ANC and provided his own legal training and reviewed his consultations with Jewish leaders in South Africa and elsewhere.

It is hardly noble of Nelson Mandela, who is exceedingly articulate about human rights conditions in his own country, to decline to comment on human rights conditions in some of the ruthless, even murderous, places where the ANC has found allies. If others followed his rule of avoiding interest in the internal affairs of other countries, moreover, his own cause would suffer. But his choices are understandable at the least as the expedient tactics of a struggling Third World liberation movement.