I wouldn't assume that flying AA means any particular percentage of people on the flight will be Americans.

My limited experience with immigration at ORD is that it's pretty quick there, especially compared to MIA. But I'm a U.S. citizen so that may not be the case for foreigners. You will have to take a train and reclear security for your onward flight, but that shouldn't take too long.

Bottom line is that 3 hours should be sufficient.