Johnson (lower back spasms) and Baker (left groin strain) will begin their stints Thursday. Soriano (left quad strain) will start his rehab Sunday.
"I can't remember the last time I was excited guys going out on rehab," Cubs manager Mike Quade said. "I've got to be honest, [I am]."
All three will play at least a couple of games in Iowa before a recall is considered. Johnson can come back as soon as Friday; Baker on Monday and Soriano June 15.
Quade looks forward to getting Johnson, who is batting .367 with two home runs and 18 RBIs, back first.
"If he's ready on his come-off day, then I'm ready," Quade said. "That's the way that goes."
Johnson has had four back-related injuries that have landed him on the DL during his career. He became a regular after Marlon Byrd was hit in the face in Boston on May 21. Eventually, playing every day caused his back to lock up on him.
The Cubs, who have lost eight straight heading into Wednesday's game against the Cincinnati Reds, have six players from their opening day roster on the disabled list.
This is Soriano's fourth leg injury since joining the Cubs in 2007. He is batting .273 with 12 home runs and 29 RBIs.
Baker is batting .368 with one home run and 14 RBIs.
Bruce Levine covers the Cubs for ESPNChicago.com and ESPN 1000.