The 3 door belts mount to a bar at the bottom of the b pillar so the lower belt fixing point can slide back and forth so the belt can still function correctly with the seat positioned far forward but the belt can still be moved out the way for rear passenger access. Remember the passenger doors on a 3 door and physically quite a bit longer than the doors on a 5 door.
The 5 door belt on the other hand mounts to a fixed point at the bottom of the b pillar.
The mounts are very different you would not be able to adapt them to fit.