My guess would be that the gs3 was using a different LTE frequency because the att version only supported 700mhz and 2100mhz where as the moto x supports 700,850,1700,1900mhz. If you your talking about another variant of gs3 and moto x say T-Mobile or sprint look up the frequency's. that is my guess in signal differences. Hope this helps.
sent from my rooted at&t moto x