AK-47 is 7.62x39 while G3 is 7.62x51 (7.62NATO). It doesn't work at all.

The G3's mechanical system is a roller-locked design that requires great calculations and design tuning to function. As far as I know even out-of-spec ammunition can be a major problem.

The Chinese don't sell 7.62NATO ammo as far as I know, but ammo factories for both calibres exist in several Third World countries. They're two of the three most wide-spread military small arms calibres and it should be no problem to get that kind of ammo.

I've seen photos of African militias mixing AK, RPK, RPG, G3 and FAL - and rarely if ever more than one spare magazine/rocket per man. FAL uses same ammo as G3.