I believe our rec's here in Ontario are N+K+S in a 2x2 should not exceed 70 lbs in a Silt or Clay, or 45 lbs in a Sand for corn. By this measure, you should be safe from a burn perspective.

However, doing this in June in warm weather, depending on your handling and metering equipment, you may hate yourself with this mix. Urea in warm weather will bridge, stick, and gum up moving parts. A coupler pounds of clay-based kitty-litter / floor-dri per ton at the mixer could be helpful. Don't let it sit out in the sun under a dark tarp...

I'd be more inclined to try to figure out a way to broadcast the N either slightly before or after planting, and put the MAP in the band. The P needs to be banded, but the N will have some mobility in the soil.

YMMV,
Luke