"I'll leave the door open for all of them," said Tomlin. "We'll see how they respond to the workload."
Of the three, Mendenhall was the biggest surprise at Monday's practice. His burst and long speed had the defensive players buzzing.
"He looked incredible," said Brett Keisel.
"I thought he looked great. He was running well, finishing runs. I'm excited for him," said Lawrence Timmons.
The Steelers' timetable did not have Mendenhall returning this early from his torn ACL only 8 months ago, but he's now in the running with Isaac Redman and Jonathan Dwyer for the starting job this week.
Wallace and Harrison are more likely to start Sunday, particularly Wallace after Tomlin on Tuesday praised his conditioning and how well he executed his assignments.
Harrison, meanwhile, complained after Monday's practice about his "legs, especially with the conditioning and everything. It's a lot harder out there. It felt like the actual first day of training camp," said Harrison, whose availability will be determined by how his knee responds to practice this week.
If Harrison does play, he'll more than likely leave plenty of snaps for Chris Carter, who replaced Harrison throughout training camp. Another outside linebacker, Jason Worilds, is also expected to play.
Tomlin also confirmed that rookie Chris Rainey will return both punts and kickoffs, but in response to the question of whether he "trusted Rainey," Tomlin said: "He will be given a return opportunity. I'm not going to go so far as to say ‘trusted.' (Laughs) He's a young guy."
Ruled out of Sunday's game by Tomlin were FS Ryan Clark (sickle cell trait), ILB Stevenson Sylvester (knee) and OG David DeCastro (knee). The Steelers can put DeCastro on injured reserve after 4 p.m. Tuesday, thereby qualifying him for a return after midseason. The Steelers are expected to sign Trai Essex to replace DeCastro, but doing so next week could save the team money as well as keep their options open for a serious injury at a more important position, like quarterback.
"There's always discussion in today's NFL whether you go with two or three. I find comfort in three," Tomlin said. "When I walk up and down these hallways and look at pictures of the '70s Steelers, they had 45-man rosters and I saw three quarterbacks on those pictures. So that's a little bit of a history lesson for me."