Hi Asce! Yes, resistances stack as far as I know. If it seems like it doesn't, let me know and I'll look into your equipment and try to replicate it so I can fix the bug. :D
Ah, okay. I had Alyssa wearing all three equipment against Garuda and she got walloped with two 200 hits. Garuda was just doing big damage all around, so I couldn't tell if the resistance weakness was stacking or not.