#3 "You Can Call Me Al," Graceland, with Chevy Chase
(Okay, it's only in the video that he's not the LEAD singer, but so what?)
#2 "Written on the Subway Wall/Little Star," Yo Frankie, with Dion
(Simon's appearance here is quite appropriate since Dion's song probably owes not a little to Simon's "The words of the prophets are written on the subway walls and tenement halls" from "Sounds of Silence.)"
and the #1 Paul-Simon-as-a-Back-Up-Singer recording is
"The Blues," Trouble in Paradise, with Randy Newman
(Randy and Paul making light of Simon's at times dour persona!)