I recently purchased a dyno zone for my son...just curious what's the best setup to convert it to a 3 piece crank?

Not so sure what you meant to ask, I'll assume it has the olskool American bb. Basically you'll need a converter kit e.g. Shadow Rant kit, and a crankset of your choice. Be wary that theres 3 sizes of spindle 19, 22 & 24.for the cranks, and the bearing (from the kit) inner side dia. shld be the same with the crank spindle dia. The assembly part shld be on the crankset manual, or youtube that there plenty of tutorials. If I remember correctly, RJ the bike guy had just the tutorial abt these conversion.

Cranksets? The higher end ones are normally 2 piece (or 2.5 some makes call it) these days, less headaches all around, just my 5cent. But there are excellent 3 piece out there; Primo, Animal, to name a few. The more famous crank arm length these days are 165 & 170mm, depending on preference & riding style. Hope that answers the question