The 3V board is actually narrower, in it's plastic holder, it's the same width. However, GGG (George) has mentioned several times there are applications where the 5V board fits, but the 3V board is more problematic.
There's always a way to make them fit, at least most of the time. However, a few times I've had to extend the wiring harnesses to make a 3V board with 5V connectors fit as the plugs are in a different location on the two packages.
For the P5a, I'd have figured a way to just use one of the replacement boards and not have to redo all the wiring. I am keeping some 5V boards in my parts box for the day when one of my 5V locomotives eats a board, that will make the fix much easier.