From the small amount of highlight tape I have seen on the prep prospects I would say:
Wheeler is a true PG, good handle and passer. Ability to shoot the 3 and finish at the basket. He is undersized which could take a bit of adjustment. Monte Morris type.
Nadolny is a combo guard who can score in waves. Tough defender with solid size for his age who compete 40 minutes. Naz or NWB type player.
Grill is a SG, who has a quick release and good athleticism and height to play some SF. Matt Thomas comp (with better hops).
While I believe Tyrese will enter the season at the PG role and will be good there, Prohm has always had a score first combo PG to run his system (Caanan, Payne, Monte, and Lindell/NWB). TH hasn't shown the scoring mentality yet (it's early) and he'll will want another option with Tre. The one thing this team could use is a true 3 pt threat, Nixon will do that but it is his last year, the hope is Lewis steps up but if not Grill would fit that role nicely.
In theory ISU could use all 3 of the prospects, but having Leech (SG/SF), Jackson (CG) lessen the need for a true guard. While I am sure Prohm would take all 3, I would guess he is prioritizing a score first PG (one of Nadolny/Wheeler) and Grill for a shooting threat.
All that said, we shouldn't get to excited and rank priorities yet, nor should we panic if ISU fails to get commitments. I'll guess we'll hear about more visits being set from transfers and even preps into May, this staff does a good job keeping their who they are in contact with under wraps from the media.