A Labor Department administrator has dismissed three out of four charges of unfair labor practices brought against the National Labor Relations Board (NLRB) by its own attorneys last December.

The NLRB is the agency responsible for enforcing labor laws in the private sector.

The NLRB Professional Association, the union which represents about 175 non-supervisory attorneys who work for the NLRB in the Washington area, had objected to a requirement initiated by the board over a year ago that new employees must pledge themselves to a term of three years with the board.

Frank P. Willette, acting regional administratro for Labor, found that the board had acted within its rights "to establish certain pre-employment requirements."

Willete found that the union had "a reasonable basis" for one charge that the Board had implemented the three-year rule without consulting the union as required under the federal law governing labor-management relations in the executive branch. The union has the right to ask for a hearing on that charge.

Tom Gagliardo, an attorney for the union, said it plans to appeal Willette's findings on the other charges to the assistant secretary of Labor, Francis X. Burkhardt.

Gagliardo said the appeal will be based on the union's claim that while the employer has the right to hire who he pleases and to set certain standards, the terms affecting a person's hiring, work and the manner of his exit are negotiable.

One union source said that there is a feeling among the members that the new board chairman, John Harold Fanning, designated this month by President Carter, "has an open mind" on the issue and that a settlement is possible.

A spokesman for the board said the new chairman intends to take a "new look" at the matter.

Under the board's three-year requirement, Gagliardo said, employees who failed to work the three years joepardized future rehiring by the board and risked the board's sending a negative letter of recommendation to prospective employers.

According to a board spokesman, each of the five board members hires employees independently and do not all use the three-year rule uniformly. Employees hired under the rule have an option to quit at the end of one year but after that are considered committed for the full three years, the spokesman said.