William E. Thomas, 18, pleaded guilty yesterday to three armed rapes, one assault with intent to kill and five armed robberies, all of them committed in the prosperous Kalorama Triangle of Northwest Washington in a two-month period last fall.

Following the guilty pleas, Assistant U.S. Attorney Jason Kogan told Judge Eugene N. Hamilton of D.C. Superior Court that the government would drop charges that Thomas committed 60 other offenses.

Thomas could receive a maximum of 15 years to life in prison for each of the crimes to which he pleaded. Judge Hamilton set sentencing for May 25 pending receipt of a presentencing report from probation officials.

Because of his age, Thomas is eligible for sentencing under the Youth Corrections Act. The maximum he could receive under that law is an indeterminate sentence running up to 20 years. However, the final determination of whether he should be dealt with under that legislation lies with Judge Hamilton.

The 69 offenses for which Thomas was indicted terrorized the Kalorama and Adams-Morgan neighborhoods. Those to which the defendant pleaded guilty an others named in the indictment occured between Sept. 30 and Dec. 2.

The indictment accused Thomas of robberies, rapes, assaults and related offenses stemming from 12 incidents involving 16 victims. In one of the cases to which he pleaded guilty, he raped, shot and stabbed a woman in her home. On the previous day, he told the court, he raped another woman at pistol point.

Thomas was arrested Dec. 7 after a clerk in a liquor store recognized him from a composite, drawing produced by police on the basis of descriptions supplied by Thomas's victims.

The composite drawings and other information from police were widely distributed in the Kalorama and Adams-Morgan areas by citizens. Police priased the spirit of cooperation and said their work would be easier if more citizens would come forward with information about crime.

Fear remained in the aftermath of Thomas's arrest. Citizens who had played a role in helping police refused to disclose their names for publication for fear of reprisals. Officials familiar with the case said yesterday police still are searching for an accomplice of Thomas.

Thomas remained impassive throughout the proceeding at which he pleaded guilty, answering questions from Judge Hamilton in a barely audible voice.

At a court hearing Dec. 13, Thomas tried to hurl himself through a third-floor courtroom window when he was ordered jailed. U.S. marshals grabbed him by the heels and pulled him back into the room.

His action, followed the revocation of his parole from a juvenile court conviction on armed robbery charges. Judge Luke C. Moore, who presided at that hearing, ordered him to St. Elizabeths Hospital for a mental examination.

Kogan, the prosecutor, said yesterday that the doctors had found Thomas fit to stand trial.