I've got a bluetooth mouse/keyboard set I use on my PS3. I know some (very few) games support keyboard/mouse use, and it seems like Mass Effect 2 did not support Keyboard/mouse on PS3.
Can I use a mouse and keyboard to play Mass Effect 3 on PS3?
I have no way of knowing for sure, as I have the PC version, but I am going to take an educated guess and say no. I say this because the PC version has no gamepad support, so if they didn't want to commit resources to gamepad support on PC I really doubt they commited resources to mouse and keyboard support on the PS3.