The MG3 is pretty old, it entered service in the german forces between 1950 and 1960. MG stands for "Maschinengewehr" (german for "machine gun"). This is an old machine gun, based on the MG42 from WWII.

Real Life vs Games:

The MG3 cannot be used by one person alone. When shooting Rambo-style it is impossible to hit something. As you see in the pictures, you need a Bi-pod or a person who acts as some kind of bipod.

Is it still in use?

Yes, even though it's very old, the MG3 machine gun is used on the Leopard 2 tank (and other vehicles).