The Congressional Black Caucus, Atlanta Mayor Andrew Young and civil rights groups have asked the South African government to commute the death sentences of three young blacks condemned because of guerrilla activity.

Their executions are scheduled for any time after today, when the deadline for clemency appeals expires.

Letters also were sent yesterday to President Reagan and Secretary of State Alexander M. Haig Jr. by the Ad Hoc Monitoring Group on Southern Africa, an informal watchdog group of legislators, asking the administration to request clemency for the three men.

The administration last month voted in favor of a U.N. Security Council resolution asking the Pretoria government to commute the death sentences out of concern that carrying them out "would further aggravate the situation in South Africa."

It is the first time in more than 20 years that the death sentence has been handed down in South Africa for a politically motivated crime in which there was no loss of life. Ncimbiti Johnson Lubisi, Petrus Tsepo Mashigo and Naphtali Manana, all in their 20s, were found guilty of treason last year for being members of a banned guerrilla movement. They also were convicted of attacking a rural police station.

Other groups which have sent messages include the National Bar Association,the Africa office of the National Council of Churches, the American Bar Association, the Lawyers' Committee for Civil Rights Under Law, and Amnesty International.