Three candidates for the post of suffragan, or assistant bishop, of the Episcopal Diocese of Washington have been nominated by petition, giving the special convention on May 31 a field of seven from which to choose.

Petitions, each signed by at least 25 clergy and 50 lay communicants, were filed this week. The three new nominees are the Rev. William M. Baxter, 63, of Portland, Maine; the Rev. Christopher Sherrill, 51, of McLean, and the Rev. William Wendt, 66, of the District.

A nominating committee had already submitted the names of the Rev. Mary Chotard Doll, 47, of Cincinnati; the Rev. Lawrence Harris Jr., 45, of Leeland, Md.; the Rev. Ronald H. Haines, 52, of Black Mountain, N.C., and the Rev. John D. Chamblin, 59, of Silver Spring.

Baxter, who retired in 1983, was rector of St. Mark's Parish on Capital Hill from 1954 to 1966. He spent four years in government service, first as director of public affairs for the Peace Corps and later as an envoy for President Johnson exploring problems of Vietnamese refugees.

In 1970, he became a partner in the Marriage and Family Institute, a counseling center based in D.C.

Sherrill is assistant rector of All Souls' Parish in the District and vice president and director for the southeastern region of THinc. Career Planning Corporation, an executive out-placement service. He has served as rector of St. Patrick's Parish in the District and as rector of St. Matthew's in Seat Pleasant.

Wendt is the executive director of St. Francis Center, which he founded in 1978 to deal with problems of death and dying.

From 1960 to 1978 he was rector of the St. Stephen and the Incarnation Parish in the District.

All seven candidates will be available for questioning at 7:30 p.m. Friday at All Saints in Chevy Chase and at 9:30 a.m. May 24 at Christ Church in Clinton.