If a rifle is permitted for hunting, semi autos can be used in MOST states. Pennsylvania does not permit them for big game hunting. You would need to check the hunting regulations for YOUR state.
The G3 is full auto, not semi-auto. Semi auto variants (such as the HK91) and clones (such as the Century Arms 'frankenguns' and the PTR-91) have been made. The G3 itself doesn't meet the definition...