I've done a little searching, not much, but it looks like the only things that are truly the same are the front lower control arms, front calipers, front rotors, front and rear sway bar links, and rear hubs (so long as the 3 has ABS). And a lot of nuts and bolts..... basically anything that has a BL or BP in the beginning of the part number crosses over.
LOOKING under a 3 and a 5, the sway bars APPEAR the same, so perhaps the part number difference is only because of different thicknesses?
I've been wanting to order a set of the MazdaSpeed sway bars for the 3, just to see if they will mount up, but that front bar looks like a PITA to swap out.