The LAS immigration/customs area is one level down from the departures gate area on the east side of T3. Gates E1-E7 can be configured for domestic arrivals with the departure gate doors open (so arriving passengers can enter the T3 departures area) or those doors can be closed and arriving passengers are forced down escalators to the CBP area after deplaning. Gates E8-E15 on the west side of T3 don't have this layout. That's why, as The_Banking_Scot observed, international flights only arrive at Gates E1-E7.
There are only three escalators that go down to the CBP area (not one per gate); Gates E1 and E2 share one escalator, Gates E3 and E4 share one escalator, and Gates E5-E7 all share one elevator. IIRC, the escalator areas are behind big walls so they are harder to see from the departure area, which is why OP may not have noticed them.