Print three separate prints and then physically align them properly. Shoot a copy neg of your composite print and then print your composite neg. You still may require an unsharp mask doing it this way. But I would try it without first.
If at all possible, I would shoot the copy neg on larger format then 35 mm.
The alternative would be a registration easel and probably pin registered negative carrier...not practical with 35 mm.
I hope that you had the camera perfectly level and plumb.