Union negotiators for about 1,400 school aides and clerical employes in D.C. public schools yesterday reached preliminary agreement with the Board of Education on a three-year contract, providing an immediate 5 percent pay increase retroactive to last Nov. 1 plus a 3.5 percent bonus.

The tentative agreement, produced after five months of negotiation, also provides a 7 percent across-the-board raise for fiscal year 1983, which begins next Oct. 1, and a raise the following year of between 6 percent and 8 percent, with the precise amount to be set according to the cost-of-living index.

In addition to the pay raises, the contract calls for optical and dental benefits for these workers--secretaries, teacher assistants, computer programmers and others--for the first time, according to George Johnson, spokesman for the American Federation of State County and Municipal Employees (AFSCME). The employes are represented by AFSCME Local 2921.

For the first time, under the provisions of the 1980 Merit Personnel Act, the employes and the school board negotiated this year a broad range of issues, including salaries and benefits.

Talks are still under way between the board and AFSCME units for contracts covering about 3,100 teachers and 1,600 workers in maintenance, engineering, food service and other support service jobs, Johnson said. Meanwhile, talks between the board and the Washington Teachers Union are being conducted by a federal mediator.