I think a lot of this hinges on Hopkins being 100% and being an all-BE guy from Day 1. If he can, Hopkins will be totally fine guarding 4s -- especially in the Big East.The best way to define what position a guy plays is by who they defend. Mitchell is by far the better perimeter defender. You want him guarding the 3 over Hopkins 100% of the time.
I think you could get away with Mitchell at the 3 if you won the guard battle, but I don't see that happening unless Darling is just an absolute stud or Jackson is stud PG. We are likely going to have to build some skills back with a third guard like Sanon or Sellers.
Which match-ups particularly concern you? I actually like Mitchell, Hopkins, Zuby against UConn's front court (Mullens/Stewart, Karaban, Reed) and Creighton's (Harper, McAndrew, Freeman). We're absolutely more athletic than the latter. After those two, just from quick glances, the other teams projected to be in the upper half have the following projected at the 3: Corey Floyd Jr. (Providence, 6'5"), Chase Ross (Marquette 6'5"), KJ Lewis (Georgetown, 6'5). I think we can use our size and athleticism (scary to have Prey on the bench as well) to really bully these teams.
Offensively, it's hard to imagine Mitchell being all that much of an offensive detriment with Zuby grabbing offensive rebounds, Hopkins being an elite college playmaker with his size, and the shooting from a combo of Jackson/Sellers/Sanon. I'd also say Mitchell has the unique ability to make his defense into offense because of his freakish ability.
Of course, Rick plays mind games and we'll probably see 10 variations of the lineup. But he knows Mitchell will need to be at the 3 against certain championship caliber teams (maybe UConn, certainly Florida w/ Haugh at the 3, Houston, BYU, UCLA, Duke)