Can I use a controller when playing on my pc

Not officially, no. However there are workarounds with 3rd party programs that many ME3 players use. There are some discussions about it about it here: http://answers.ea.com/t5/Mass-Effect-3/Mass-effect-games-and-controller-support/m-p/485832

http://answers.ea.com/t5/Mass-Effect-3/XBOX-Gamepad-Native-Support-On-PC/td-p/1044432

http://answers.ea.com/t5/Mass-Effect-2/PC-360-controller-for-Mass-effect/m-p/1766209