Yoko who sang on "Give Peace a Chance"

ONO

This avant-garde artist and peace activist was married to a famous Beatle and often collaborated on experimental music projects. Her first name is just three letters long.

This clue refers to Yoko Ono, the artist and musician famously married to John Lennon. She contributed vocals to the 1969 anti-war anthem "Give Peace a Chance," which was recorded during Lennon and Ono’s "Bed-In for Peace" protest. The answer is simply her last name, "ONO," a common crossword shorthand for her. The clue relies on recognizing her cultural impact and connection to the song.