The Washington Post Co. yesterday named three new outside members to its board of directors -- Barbara Scott Preiskel, William J. Ruane and George W. Wilson.

Preiskel, who is the first black to serve on the board, is a former senior vice president and general counsel for the Motion Picture Association of America, where she worked for 25 years. She also serves on the boards of American Stores Co., Textron Inc., Levi Straus & Co., R.H. Macy & Co. Inc., General Electric Co., Massachusetts Mutual Life Insurance Co. and as a trustee of the Ford Foundation.

William J. Ruane, 59, is chairman of the board of Ruane, Cuniff & Co., an investment managment firm, and chairman of the Sequoia Fund, an open-end mutual fund.

George W. Wilson, 48, publisher of the Concord (N.H.) Monitor and president of Newspapers of New England, once worked in Washington as a copyboy for The Post and for Sen. Strom Thurmond (R-N.C). Newspapers of New England owns or has a direct interest in several northeastern newspapers.

The three new directors increase the total size of the board to 12.Nine of those directors are outside directors.

However, Post Co. board member Warren Buffett is expected to resign when the merger of Capital Cities Communications and American Broadcasting Cos. is completed. Buffett, who will be a director of the newly merged companies, is resigning because of Federal Communications Commission rules that restrict cross-ownership of media.