If you'll be playing Mass Effect 3 on PC, you'd better be comfortable with a mouse and keyboard. BioWare community manager Chris Priestly responded to requests on the company's forums with a definitive no to official support for game controllers, though he notes they might work anyway.
On the bright side, if you're planning on playing Mass Effect 3 on the PC, it's probably because you played the first two games on the platform as well. Neither officially supported controllers, so this won't be much of a change for you. If you're a newcomer thinking about jumping into the PC version for the first time, a demo hit Origin yesterday. You can try it out, if your PC has the required juice, to test the M/KB controls yourself.