It should be Butler.
Trooper 7 and 3 are both based at APU 3 most of the time. Trooper 7 is the spare ship. All of the helos can be temporarily operating from other APUs as they are shifted around to cover maintenance down time. If 7 is out, though, it is normally because another is down for service. Both planes are at APU 3 typically. Normal assignment is:
871ST Trooper 1 APU 1 – Reading Airport
872ST Trooper 2 APU 2 – Hazleton
873ST Trooper 3 APU 3 – Capitol City Airport
874ST Trooper 4 APU 4 – Altoona Blair Co. Airport
875ST Trooper 5 APU 5 – Latrobe, Palmer Regional Airport
876ST Trooper 6 APU 6 – Franklin, Venango Regional Airport
877ST Trooper 7 APU 3 – Capitol City Airport
878ST Trooper 8 APU 3 – Capitol City Airport
879ST Trooper 9 APU 3 – Capitol City Airport