Mary Treadwell, testifying for a third day at her fraud and conspiracy trial, yesterday accused Robert E. Lee, the former general manager of the real estate firm she headed, of widespread misuse of the company's funds in the mid-1970s.

In testimony about a variety of financial transactions, Treadwell told the federal court jury of eight women and four men that it was Lee, not she, who tapped the accounts of P.I. Properties Inc. for cash withdrawals, personal expenses and use as collateral for loans.

Treadwell also disputed prosecution testimony that she suggested allowing the Department of Housing and Urban Development to try to foreclose on the mortgage on the dilapidated Clifton Terrace apartment complex owned by P.I. Properties so that the firm could pay off its 20-year loan in one lump sum and turn the project into condominiums.

The 42-year-old Treadwell said it had been Lee who "shopped Clifton Terrace around to private lending institutions" in hopes of finding financing to pay off the HUD mortgage.

She said she took HUD's threat to foreclose on the Clifton Terrace mortgage "very seriously" and sought to alleviate the problems that P.I. Properties was encountering with the federal agency over the firm's ownership and management of Clifton Terrace. HUD eventually foreclosed in August of 1978.

Lee has pleaded guilty to conspiracy in the case. Treadwell has conceded that a variety of improprieties were committed in P.I. Properties' operation of Clifton Terrace, but said she did not commit them. However, she said she knew that security deposits of Clifton Terrace tenants were used as collateral for two loans that P.I. Properties obtained for a separate Treadwell-run company. Such use of security desposits is illegal under D.C. law.

Lee has not testified in the trial, but is expected to be called as a rebuttal witness after the defense completes its case.

Treadwell's testimony about her desire to keep HUD from foreclosing on the Clifton Terrace mortgage was an attempt to undercut notes, already introduced as evidence in the case, that she made of a June 1975 meeting that indicated she might have been thinking of selling Clifton Terrace before the 20-year mortgage was paid off.

Under HUD rules, if the agency attempted to foreclose on the mortgage, the buyer could keep from losing the property by paying off the entire mortgage and would no longer have to follow HUD rules to provide low- and moderate-income housing. The provision, according to one HUD attorney who testified for the government, was a "legal loophole."

In her notes, Treadwell had written: "20-year time reduced. Don't mention legal loophole."

Treadwell said that she only belatedly learned of Lee's use of P.I. Properties' accounts for his own use. She said that she put $15,775 back into Clifton Terrace's security deposit account after she learned that Lee had used it as collateral for a loan.

Eventually, Treadwell said, she fired Lee in August 1977 and told him in a letter that "your conduct has been unprofessional and detrimental to the organization."