Is there any bonus content I would get for acquiring Mass Effect 3 on PC over say PS3, or Xbox 360 over PC? As lots of games seem to offer content exclusive to one that you can't get on another (Call of duty getting DLC a month earlier on xbox being a good example) and if so what is it? And does it make much of a difference?
The Xbox 360 version has Kinect support, which is exclusive to this version.
The PC version requires Origin, which is a bit stricter/more intrusive DRM than either of the other two platforms. However, this also means it is digitally distributed as opposed to needing to purchase a physical disc.
The Playstation 3 never had the original Mass Effect, (at that time, the game was an Xbox 360 platform exclusive property on consoles) so you may be at a slight disadvantage if you would have made different choices in the original game that effect the starting point for this game. For instance, your choices in ME1 near the end of the game change certain War Assets in this game.
Aside from these differences, I'm not aware of any platform-specific DLC exclusives for any of the three versions.
There were a few pre-order bonuses which could be considered platform exclusives, for instance preordering the PC version via Origin would have secured you a AT-12 Raider, which is a unique shotgun.