What gun did the machine gun that the MG 3 is derived from replace?
Rheinmetall MG 3:  The weapon's design is derived from the World War II era MG 42 universal machine gun that fired the 7.92×57mm Mauser round. MG 42:  It was intended to replace the earlier MG 34, which was more expensive and took much longer to produce, but both weapons were produced until the end of the war.
MG 34