Negotiators for the D.C. Board of Education and the city's 4,500-member teachers union yesterday reached a tentative agreement on a three-year contract that would provide annual pay raises of roughly 3 percent to 4 percent and that would leave the school day and school year at their current lengths.

The settlement, which must be approved by members of the Washington Teachers Union and the school board, leaves intact the current provisions for dental and optical benefits and other contract provisions, according to teachers union president Harold Fisher.

The teachers' wage settlement, which begins with a one-time 3 percent bonus rather than an annual salary raise in its first year, follows a pattern set in settlements in the past year with other municipal unions representing 20,000 blue- and white-collar workers, police and firefighters.

The bonus, which has become increasingly popular in contract negotiations, would save the city a considerable amount of money because it would freeze teachers' base salaries in the first year and because many benefits such as vacation, overtime pay and raises are calculated from base salaries.

A school board spokesman yesterday confirmed that an agreement has been reached but declined to comment on the terms pending its approval. Fisher said teachers will vote on the pact by Monday.

Last week, the board and the union declared their 10-month talks to be at an impasse and asked the D.C. Public Employee Relations Board to appoint a mediator. Fisher said then that a primary roadblock was the board's insistence on lengthening the school year by 10 days and extending the teachers' workday a half-hour to 4 p.m. Both provisions were dropped during negotiations, Fisher told a teachers meeting yesterday.

Fisher said the board's wage offers, which began with a 2 percent bonus, were inadequate if the school system wants to attract and retain good teachers.

District teachers, who earn an average of about $30,000 annually, have tentatively agreed to considerably less than their initial proposals but slightly more than the board offered. The union had sought raises totaling 27 percent over three years, while the board countered with an offer of a 2 percent bonus in the first year, followed by raises of 2.5 percent and 3 percent.

Teachers would receive a 3 percent bonus for fiscal 1984, followed by a 3.5 percent wage increase for this year, a 4 percent increase in October 1986 and a 1.5 percent increase at the end of fiscal 1986. The previous teacher contract was signed in 1982 and expired in April, and teachers had not received a raise for 1984 while the negotiations continued.

The current raises are considerably below those of the previous contract, in which teachers received raises of 6 percent to 8 percent a year.