Why would you need to check in at A? I guess I've never tried to check in at B/C for an international departure (in fact, I don't think I've ever even checked in at a counter at all for international; never had checked bags out of PHL when traveling internationally), but I've never seen any indication that AA cares where you check in. If B/C check-in is more convenient, I'd definitely try checking in there no matter the destination.

Of course, assuming you can check in at B/C, you can clear security at B and never have to go outside (either outside in the weather or outside security) to get to A gates.

Lovethecabin is flying through MIA, so the PHL-MIA flight could go out of any of A, B, or C.