You can definitely use the Admirals Club at ORD in T3 but please keep in mind that as of now, US flies out of T2. You can connect between T2 Concourse F and T3 Concourse G without having to reclear security. I'd suggest using the Admirals Club in Concourse G as it's much more modern than the one between the H-K connector and is also going to be closer to your departure out of T2.

Again, I don't know when you're departing so make sure that US is indeed flying ORD - PHL out of T2 before heeding this advice.