A U.S. judge yesterday ordered former U.S. Rep. Charles C. Diggs Jr. to report to federal prison later this month to begin serving a three-year sentence for his 1978 conviction on fraud and kickback charges.

U.S. District Judge Oliver Gasch rejected a plea by Diggs and his attorney that the former House District Committee chairman be spared the prison term or that it be reduced.

"I have acknowledged my guilt and wrongdoing . . . and I'm here to further acknowledge my misconduct," Diggs said yesterday in his last-chance plea to avoid the prison term. The Supreme Court last month refused to hear his appeal from his October 1978 conviction on 29 criminal counts.

Gasch said he did not believe a long prison term was necessary in Diggs' case, but "in the interest of uniformity that some term should be imposed."

Diggs, a 13-term Democrat from Detroit, will report to prison after July 28, and has requested, with the judges's approval, that his term be served at Maxwell Air Froce Base outside Montgomery, Ala. Diggs technically is eligible for parole the first day he enters prison, but in reality will probably serve about one-third of the three-year term.

Diggs' attorney, David Povich, said Diggs should not be sent to prison because he had been financially and personally ruined by his ordeal and conviction. He is paying more than $40,000 to Congress at the rate of $500 a month to repay the money he acknowledges having mishandled.

Diggs was convicted of inflating the salaries of five staff members and using the additional money to help pay his own office and personal debts.

He has been free pending his unsuccessful appeals. In the meantime, he has formally apologized to Congress for his wrongdoing to avoid expulsion from that body and was publicly censured there in July 1979.

He also gave up his power bases as chairman of the Foreign Affairs subcommittee on Africa and the House District Committee. In May of this year, he announced he would retire from Congress at the end of this term, and the next month he announced his resignation.

"There is evidence this man accepted responsibility for what he has done," Povich said, noting Diggs had sold his house to pay off his personal debts. "There is no necessity to further punish him."

Diggs first came to Congress in 1955, and was founder and ranking member of the Congressional Black Caucus. An undertaker by profession, he also had served in the Michigan State Senate before being elected to Congress.

Diggs was reelected by a landslide about a month after being convicted of the criminal charges in 1978. He earlier had insisted he also would seek reelection again this year, but by May it was clear he had become a liability to the Detroit Democratic political establishment.

He was only the second congressman this century to be disciplined by the House for misconduct.