X3 for a 3/8 for anything under 3/4" impacts... Hi-flow couplers like Bob mentioned are a plus..++
Keep in mind that Milton does not recommend using the type V couplers directly connected to an impact wrench.. Use a whip or "leader hose" between the impact wrench and the male fitting... There are two benefits: your impact wrench is less clumbersome without that coupler sticking out, and the vibration and impact will not shake the **** out of the coupler..