Sen. Charles McC. Mathias (R-Md.) has recommended three candidates to President Reagan to succeed J. Frederick Motz as U.S. attorney for Maryland.

They are Washington attorney Breckinridge L. Willcox, 40, a former Mathias aide who lives in Potomac; Assistant U.S. Attorney Ty Cobb, 34, of Silver Spring; and Baltimore attorney Richard D. Bennett, 37, a former assistant U.S. attorney.

Motz will be sworn in Monday as a judge in federal district court in Baltimore.

Bennett has been active in the Maryland Republican Party, as head of Reagan's reelection campaign in Baltimore and as an unsuccessful candidate for the state Senate in 1982. He was not among Mathias' original choices, according to Rep. Marjorie S. Holt (R-Md.), but his name was included after Holt, Rep. Helen D. Bentley (R-Md.) and top state party officials made a direct appeal to Attorney General Edwin Meese III and Fred Fielding, the White House counsel.

Mathias interviewed 10 applicants last month, including Bennett. He then met with Holt and Bentley, and told them that his choices were Willcox and Cobb, according to Holt.

Mathias spokeswoman Ann Pincus said that Mathias "was heavily lobbied," but that he might have included Bennett's name anyway. She said Mathias always intended to send three names to the White House.

Allan C. Levey, the state party chairman, said GOP officials support Bennett because "he's as qualified or more qualified than the others, and he's been active in the party."

Levey said that Bennett might run for office someday and could be helped by such a high profile position.

Willcox worked for Mathias from 1973 to 1975, and spent the last 10 years in the criminal fraud division of the Justice Department. Cobb is an assistant U.S. attorney serving as chief of criminal cases.