Accountant Ronald S. Williams, indicted on charges he conspired with his estranged wife Mary Treadwell and three other officials of a Youth Pride Inc. spinoff company to defraud the government, pleaded guilty to a lesser charge yesterday and agreed to testify for the prosecution.

Williams' plea brings to at least three the number of persons with inside knowledge of the finances of P.I. Properties Inc., the spinoff company, who are cooperating with prosecutors. Johnny Mickens III, Treadwell's former chief financial officer, has also entered a guilty plea and agreed to testify. Zellene Laney, former P.I. Properties bookkeeper, has already had extensive conversations with prosecutors.

Treadwell and the three other defendants, Robert E. Lee, Treadwell's sister Joan M. Booth and Charles W. Rinker Jr., have pleaded not guilty to charges that they defrauded low-income tenants and the federal government of thousands of dollars between 1974 and 1978 by using P.I. Properties to obtain government money for personal expenses and to fund their own profitmaking enterprises.

Covington & Burling, one of the largest and most prestigious law firms in Washington, informed Judge John Garrett Penn yesterday that it has agreed to act as court-appointed defense lawyer for Treadwell. That means that any legal fees paid to the firm will come from the federal government, under the federal Criminal Justice Act, at no cost to Treadwell.

However, Penn could order her to make some contribution to the defense costs if she were able.

Penn granted Treadwell's request for a court-appointed lawyer after she contended that she could not afford the sophisticated legal help she believed she needed to defend herself against the criminal charges.

Penn has not yet entered a formal court order appointing Covington & Burling to the case.

However, Peter J. Nickles, a partner in the firm, said that he met with Treadwell for about an hour Monday and informed Penn yesterday that he would accept the appointment. "The court asked if I would take it . . . I did not volunteer," Nickles said in a telephone interview.

Prosecutors said yesterday they were concerned about Covington's representation of Treadwell since former U.S. attorney Charles F.C. Ruff, who supervised the Pride investigation, is now a partner at that law firm. Nickles said that the firm would take steps to assure that Ruff would be excluded from the case, but prosecutors said they are considering whether to protest the appointment.

Nickles is perhaps best known in the federal courthouse here for his successful representation of inmates at Lorton Reformatory who had complained in civil lawsuits about security and living conditions at the institution.

Treadwell said yesterday that she was "very pleased . . . that the court has responded in terms of giving me adequate resources" to defend herself. She said she was aware that Williams had entered a guilty plea yesterday, but added: "I don't think he did anything wrong."

Williams, who lives in Plainfield, N.J., faces up to five years in prison, a $10,000 fine or both after pleading guilty to giving false statements to federal investigators. Penn scheduled sentencing for May 24.

At the hearing yesterday, Williams said that in November 1976, when he began an audit of P.I. Properties records for the years 1974 and 1975, he understood that the business was being "pressured" by the Department of Housing and Urban Development to account for expenditures of federal money.

Williams told Penn he discovered the books and records were in disarray and were "unauditable." Williams said he also told Treadwell, Booth and Lee that he believed that HUD would consider some of the transactions to be illegal.

Williams said he continued to try to conduct an audit of the records until July 1977, when he informed Treadwell in a letter that it could not be done, but that he would continue to try to design an accounting system for the company.

Treadwell and Williams were married in September 1977 and separated in 1978. P.I. Properties managed the Clifton Terrace apartment complex in Northwest Washington and two other low-income housing projects in the city.

Assistant U.S. Attorney Stephen R. Spivack told Penn the government's evidence would show that after Williams examined P.I. Properties financial records he told company clerical employes that "someone is going to jail." Spivack said Williams allegedly directed employes to the documents that "would cause trouble."

Spivack said the evidence would show that Treadwell and Lee told HUD officials that Williams' firm was conducting an "independent" audit of the business' records. The government contends that Treadwell and other P.I. Properties officials used the firm to try to delay HUD auditors from examining their operation.

Spivack said the government could prove that the audit was not "independent" since Williams had been Treadwell's "boyfriend" since early 1976. Moreover, Spivack said the government would prove that the "firm" that Williams' operated consisted of himself and two friends and that its "first and last endeavor" was to try to audit the P.I. Properties books.