how do i use a controller with mass effect 3 on my pc?

Unfortunately there's not an official way to use a gamepad with ME3. However, there are various 3rd-party programs that can be used to enable gamepads in games that don't have pad support.  I don't think I can make any recommendations or endorse them, but you can find tutorials in various places, or perhaps a member of the community can make a recommendation.

It's discussed in this post which you might hind helpful.

Only one I've ever heard of anyone using is "Xpadder".