Keyboard and mouse is where it's at for PC Mass Effect 3, much like it was with the previous titles. They "may or may not work," but BioWare doesn't officially support them.
None of the Mass Effect games on PC have supported game controllers and the same goes for Dragon Age. BioWare is adamant they've all been tailored to meet the needs of PC.
"The PC version does not support controllers," BioWare's Chris Priestly. "They may or may not work, but they are unsupported by the game." Mass Effect 3 releases on Xbox 360, PS3 and PC March 6th in the US, 9th in Europe.