With more than $3 million spent in its investigations thus far, the House Assassinations Committee has announced plans to wind up its work with nearly 40 days of public hearings on the murders of President John F. Kennedy and the Rev. Martin Luther King Jr.

Chairman Louis Stokes (D-Ohio) said the committee expects to begin Aug. 14 with five days of hearings on King's assasination in 1968 and then convene throughout September on the president's death in Dallas in 1963.

The House panel plans to meet again in November and December to conclude its inquiry into the King murder - for which James Earl, Ray is serving a 99-year term in a Tennessee state prison.

According to informed sources, it may take more money than the committee now has to fulfill the schedule. The assassinations panel reportedly has been planning to cut some 30 staffers from the payroll and intending to seek a supplemental appropriation of several hundred thousands dollars in an effort to meet its end-of-the-year deadline.

Stokes declined to comment on those reports.

The House has already voted more than $1.5 million for the assassinations inquiry since it was first approved in September of 1976. After an initial series of controversies that nearly wiped it out, the committee resumed its work in secrecy last year.

The bulk of its expenditures that far have been on staff salaries and travel. The committee has held only two public hearings at which it sought substantive testimony. On both occasions, the witnesses invoked their First Amendment rights against self-incrimination.

As a result, the forthcoming series of hearings will constitute the first public glimpse into the quality of the committee's work.

In announcing the tentative schedule, Stokes emphasized that the committee's fiel dinvestigations would continue "up to and throughout the public hearings." For the King inquiry, he said cautiously that "an effort will be made to secure the testimony of Mr. (James Earl) Ray" as part of the eattempt to determine his "involvement, if any . . . ."

After the end of the hearings Dec. 1. the committee plans to meet in public again for five days starting Dec. 12 "to discuss an dresolve the issues that have been presente dto it," Stokes announced.