Painter said in a text message they would return to school. The NCAA's deadline to withdraw from the NBA draft was at 5 p.m. ET Saturday.
Johnson and Moore will join Robbie Hummel on next year's likely Big Ten favorite.
Hummel is recovering from an ACL surgery. All three have been members of an all-conference team at some point in their careers. They give the Boilermakers a trio of elite seniors at a power-six conference school in this early entry era.
Johnson was projected as a second-round pick. Moore ran the risk of not getting drafted.
Andy Katz is a senior college basketball writer for ESPN.com.