Two Democrats and a Republican, all from Fairfax County, have added their names to the already crowded list of candidates seeking party nominations for the Virginia House of Delegates.

Among the three is incumbent Del. Dorothy S. McDiarmid, a Democrat and the senior member of Northern Virginia's legislative delegation. McDiarmid announced last week she will seek a 10th term in the House, vying for one of three seats in the newly created 50th District, which includes Fairfax City and northwestern Fairfax County.

In two other Fairfax County House districts, Democrat Flora Crater announced she will seek a seat from the 49th District, covering Falls Church and northeastern Fairfax, and Republican Frank Medico said he will run for a seat from the 52nd District, covering the southwestern section of the county. Both districts will send three members to the House of Delegates.

McDiarmid, who was first elected in 1960, said she will use her positions on the House Appropriations Committee and Education Committee to monitor the effects of Reagan budget cuts on state programs. She said she would fight any attempt to transfer fiscal responsibility for affected state programs to the local level.

A longtime advocate of women's rights, McDiarmid sponsored a resolution during the past legislative session calling for ratification of the Equal Rights Amendment. She added she would continue to fight for its passage.

Joining McDiarmid in support of the Equal Right's Amendment is candidate Crater. Active in the women's movement since 1970, Crater is the founder of the Virginia Women's Political Caucus and author of "The Almanac of Virginia Politics." This is Crater's third bid for elected office. In 1973, she ran unsuccessfully for lieutenant governor, and in 1978, lost in her campaign for the U.S. Senate.

Republical Medico is a former assistant director of the U.S. General Accounting Office and has been active in civic affairs in the Mount Vernon area. Medico said he believes the state legislature should review statutes governing sentencing for violent crimes; in particular, he advocates mandatory sentencing for persons convicted of violent crimes or those convicted of using a handgun during the commission of a crime. He said those convicted of less serious crimes should be offered alternatives to imprisonment.

Party primaries have been set for Sept. 8 and the general election for Nov. 3.