Only 3? How difficult! I'd problably choose different 3 each day. I refuse to choose 3 and I'll only list some of my favourites as come to my mind at this moment.
composer: Mozart (no problem here to choose a single one)
vocalists: Ella Fitzgerald, Jamie Cullum, Robert Smith
Talking Heads, Devo, Bowie, Ella Fitzgerald, J. Coltrane, Miles Davis, Brian Eno, L. Armstrong, the Cure, Down to the Bone, De-Phazz, Tuxedomoon, Jamiroquai, Steely Dan, Ryuichi Sakamoto, Béla Fleck, Stevie Wonder, NIN, Sonny Rollins, Marylin Manson, Brand New Heavies, Béla Fleck, Hendrix, Incognito, Anastacia, Dire Straits, Yes, Prodigy, Chemical Brothers, the Residents, Rinocerose, David Sanborn, John Scofield, Shakatak, Simply Red, Frank Sinatra, Sting, Incubus, Quincy Jones, Metropolitan Jazz Affair, John Cale, David Sylvian, Cassandra Wilson, ...