Hi Sojin,
I've gone through SAS: Zombie Assault 3 developer's website to find any solution for your zombie problem. Many users reported this co-op multiplayer mode problem and game developer Ninja Kiwi stated that the problem is fixed now. You have to update your games to fix this zombie problem.
Thanks.
Anybody has any fix for this zombie problem, plz help. Or cheats/glitches for the guns and weapons?