Rep. Charles C. Diggs Jr. (D-Mich.) was sentenced to a maximum of three years in prison yesterday for illegally diverting more than $70,000 of his congressional employes' salary to his personal use.

In pronouncing sentence, United States District Court Judge Oliver Gasch invoked a provision of federal laws that leaves to the U.S. Parole Commission the discretion to determine what minimum length of time Diggs must serve in prison before being paroled.

Gasch noted that Diggs would have "no minimum term" to serve before being eligible. In practice, however, according to Justice Department officials, Diggs probably will serve about 18 months before being released.

Diggs, calm and somber throughout the proceedings, described his trial and conviction last month as a "very devastating experience," in asking Gasch not to sentence him to jail. Diggs, the senior black member of Congress, faces the loss of his House District Committee and International Affairs African subcommittee chairmanships when the House convenes in January.

"I know the conviction has been a very painful experience for me personally and professionally," Diggs said.

Gasch took the unusual step of inviting Diggs' defense lawyer. David Povich, to come back sometime in the next four months to seek a reduction of the sentence imposed yesterday. Gasch called attention to a report filed by federal probation officers noting that Diggs had a personal debt of $174,000 and a newspaper report last week that Diggs had sold his Capitol Hill home for $235,000.

"I'm not saying that that [disposition of Diggs' personal debt] would influence the court," Gasch said, "but I'd like to know about it." According to one informed source, it is highly unusual for a judge to invite a defense attorney to return seeking a reduction of sentence.

Povich said yesterday that he will file a notice of appeal with the United States Court of Appeals seeking to overturn Diggs' conviction before the statutory 10-day limit expires.

His conviction, Diggs said, "has shaken the faith of many people who identified with me and the issues that have been the hallmark of my years of service." Diggs asked Gasch for the "freedom to redeem myself for the remainder of my public service."

At least one other congressman, Thomas L. Jane (D-Mass), was convicted of a crime while serving in Congress and served a jail term while in Congress. Lane served four months in prison for income tax evasion in 1956 and went back to Congress to serve another six years.

Elected to a 13th term on Nov. 7, Diggs appears intent on taking his seat. The House may not under a 1969 Supreme Court ruling involving the "exclusion," of Rep. Adam Clayton Powell (D-N.Y.) for alleged misconduct, refuse to seat Diggs because of his conviction. Under the ruling, a member may be excluded, which requires a simply majority vote, only for reasons of eligibility - which involves only age, residency and citizenship.

Diggs could be expelled by a two-thirds vote of the House, although no member has been expelled since the time of the Civil War and then for questions of loyalty.

Diggs voluntarily stepped down for the remainder of the current Congress from the chairmanship of the House District Committee and the International Relations subcommittee on Africa. An aide said yesterday he will have a statement tomorrow concerning the chairmanships in the next Congress, which begins in January. It is considered unlikely according to informed sources, that the Democratic cacus would let Diggs continue as chairman of either committee even if he chooses to make an issue of the matter.

Although Diggs could continue as a member of Congress even while in prison, he could not vote in committee or in the House without being present. He would receive no pay as a member of Congress while in prison.

Besides a jail term, Diggs also faces the possibility that the federal government will seek - by civil action - to recover some or all of the money he diverted from Congressional employes' salary to his personal use.

Diggs 55, inherited from his father a prosperous funeral home - the House of Diggs - that served both as a source of income and a political power base in Detroit. As his time became more occupied by his job in Washington, however, Diggs was able to give less attention to business matters in Detroit.

Twice divorced, Diggs found himself faced with mounting expenses to pay alimony and to support his six children, according to trail testimony, at the same time that his income from the House of Diggs was declining. In 1973, he began increasing the pay of certain employes in his Washington and Detroit offices and diverting the surplus funds to his own use.

Diggs's lawyers last week filed a memorandum asking Gasch not to sentence Diggs to jail but rather to combine probation with a fine and some form of public service such as a minimum obligation to speak to high school students on subjects of the court's choice, such as the criminal 'justice system.' Diggs slipped out of the court yesterday without talking to reporters.

Justice Department prosecutor John I. Kotelly, who yesterday asked Gasch to sentence Diggs to some period of incarceration, appeared satisfied with the sentence. Asked how he felt about Diggs being re-elected to Congress after his conviction, Kotelly said, "I don't live in Detroit. He's not my congressman."