If in fact this is a split rim, you're correct that splitting it will release the bead; if there's even a few pounds of pressure left in the tire, it could also be dangerous (picture getting that rim in the face if one bolt fails after you have the other two nuts off).
Split rims are still seen occasionally on truck tires (older ones), made to ease installing the bead on the rim. What you have there, however, looks like a two-sided steel spoke rim. The three lugs hold the wheel to the axle spider or brake drum, and removing those bolts should just drop the wheel off the kart.
The solution to your stem accessibility problem is most likely to take the wheel off the kart, take it anywhere that installed tires, and ask them to replace the stem with a bent extended stem, which will protrude from the wheel and give you easier access. You'll probably have to pay a mount/dismount charge, as well as purchase the new stem(s), but then you'll have easy access to keep your tires correctly filled.