Mafia 2 has a limited story in a open world game with absolutely nothing to do in the open world its just background for when you are going from point A to point B.
Medal of Honor was a somewhat poorly received shooter with a forgettable story and generic gameplay. A good first attempt at being a CoD like game but why get it when you can just get CoD and have the CoD experience?
FNV is more of a RPG then shooter. You can be dead on a guy and miss. It is the only out of the 3 that has a decent story and replay value. Has some DLC out to, the latest of which is really good.
If you like RPG's and want a gun in it get FNV, if you want CoD like experience half baked get MoH, if you want just a basic story with action gun based gameplay get M2.