A Reston man, sentenced to two life prison terms on rape charges, pleaded guilty yesterday to a third rape in return for exemption from a new Virginia law that requires three-time convicted felons to serve their full sentences with no chance of parole.

Under a plea agreement that still must be approved by a Fairfax County Circuit Court judge, Raphael J. Boyd, 23, would be eligible for parole in 20 years despite a third life sentence to be imposed in connection with the latest rape case, according to prosecutors and his court-appointed lawyer.

"It's inconceivable to me that he Boyd would be considered for parole after serving the minimum amount of time," said Assistant Commonwealth's Attorney R. Dean Kidwell.

Kidwell said he accepted the plea bargain because of "the risk of a jury finding him not guilty" and because of the weight that three life sentences would carry when Boyd becomes eligible for parole.

Boyd pleaded guilty yesterday to raping a 24-year-old McLean woman as she returned to her apartment about 2 a.m. last Nov. 27 and robbing the woman of $6. Prosecutors recommended a life prison term on the rape and a 20-year sentence on the robbery charge. But under the plea bargain, the third rape conviction would be considered part of a "common scheme" -- one of a series of similar events -- that would prevent Boyd from being subject to the new law.

Both prosecutors and defense attorneys said that the rape of the 24-year-old woman was similar to two other rapes committed by Boyd during a six-week period, one of which occurred five days after the Nov. 27 rape.

Prior to yesterday's plea agreement, Boyd had been sentenced to a total of two life prison terms plus 101 years in prison on two rape and several robbery charges. Judge Johanna L. Fitzpatrick set sentencing for Oct. 4 on Boyd's latest plea.

Boyd, who would have been the first person in the state to be sentenced to life in prison without chance of parole under the new law, still could be subject to the statute. He faces trial on rape, robbery and abduction charges on Oct. 31 in Alexandria in connection with an attack on an Alexandria woman in February.

Boyd was arrested during an investigation of a series of rapes in Reston and western Fairfax County, one of the most intensive and costly investigations conducted by county police in recent years. Police announced in March that they had ended their investigation with the indictment of Roland V. Thompson, 26, in connection with an attack in January on a Vienna woman. A jury convicted Thompson and sentenced him to 43 years in prison. Boyd was arrested several weeks later.

Judge Fitzpatrick has the option of rejecting yesterday's plea agreement and allowing Boyd to stand trial. Under the conditions of the arrangement, the 20-year term on the robbery charge would be served concurrently with the life sentence.