Rep. Charles C. Diggs Jr. (D. Mich.), sentenced to three years in federal prison for illegally diverting congressional employes' pay to his personal use, asked the House speaker yesterday to relieve him of his duties as House District Committee chairman in the next Congress pending outcome of his appeal.

At the same time, however, Diggs made clear in a letter to House Speaker Thomas P. (Tip) O'Neill Jr. that the conviction and sentence "notwithstanding, I am a candidate to continue my chairmanship of the International Relations Subcommittee on Africa" and that he intends to participate in House affairs as a full member.

Unless the House Democratic Caucus strips Diggs of the subcommittee chairmanship, his position would be determined by the full committee's other Democratic members when they meet in January. Interviews with several of them yesterday found either no sentiment for Diggs' removal as subcommittee chairman or a feeling that any judgment now is premature.

Rep. Jonathan B. Bingham (D.N.Y.) said he could not see the issue arising concerning Diggs' continuation as subcommittee chairman. Diggs' conviction, Bingham said, was "unfortunate, to say the least, but I don't think it conflicts with his continuing to do a good job with that subcommittee."

Rep. Benjamin S. Rosenthal (D-N.Y.) said any judgment is "premature at this point." Rosenthal said he expected the matter to be discussed "at great length" by the full committee in January.

Rep. Peter A. Kostmayer (D-Pa.), a freshman who tried to have Diggs stripped of his chairmanships when he was indicted last spring, said that he felt "strongly that a person convicted of a felony should not serve in the House, but I'm in a quandary as to what to do pending appeal."

Kostmayer said that he would "probably vote to expel," a step that would require support of two thirds of the House membership and a move that seemed unlikely.

In his letter to O'Neill, Diggs recommended that the District committee chairmanship be given to Rep. Ronald V. Dellums (D-Calif.), the next senior member.

By asking O'Neill to remove him from consideration for the District committee chairmanship - an issue that would come before the Democratic caucus - Diggs apparently hopes to avoid a bruising and probably losing battle. The question of his subcommittee chairmanship, under current rules of the Democratic caucus, would not come before the caucus unless some other Democrat chose to bring up the matter.

Diggs voluntarily relinquished - for a matter of days - his two chairmanships after his conviction last month just before Congress adjourned. He noted in his letter to O'Neill that House rules state that a member convicted of a crime carrying a prison sentence of two or more years "should refrain" from participating in the business of the House and committees "unless or until" the conviction is reversed, a pardon is granted or "he is reelected to the House after the date of such conviction."

Diggs was convicted last month of 29 counts of mail fraud and making false statements in connection with the diversion to his personal use of more than $70,000 in congressional pay of his staff employes. He is free pending resolution of his appeal, a process that will take a year of longer.

Diggs was reelected with more than 80 percent of the vote in his Detroit district on Nov. 7. "The faith in my integrity expressed by this overwhelming margin is both humbling and personally satisfying," he said in his letter.