three samsung s3 not working according to specs

In March 2015 I bought three brand new Samsung s3 phones on mtn contract. Each phone freezes , does not show contacts , doesnt allow calls made or received, constantly have to restart all the phones to function.i have taken them to Samsung store to be checked. Each has been re updated etc. Still the same problems persists. I will never buy a Samsung phone again. Are there any other people experiencing this problem. Many times I just wanna throw it against the wall due to frustration.