No, the HK G3 and derrivatives are not legal in CA unless it was registered to you prior to Jan 01, 2001. Since you are asking, I'll assume that's not the case.
I suppose you could build a clone without a pistol grip and muzzle device, but I don't see that as being particularly useful.
There are only a few CA legal HK long guns out there. The HK SL6 (5.56mm) and SL7 (7.62) are still legal at this time. Some of the HK 630s and 770s have flash hiders, so they have to be taken case by case.
