There are two sets of A-B F3's offered. Both will get you an ABBA. Also offering a single F3A. To get an extra F3A, you need to buy SLSF 5010. Then, with an F3AB set, you can have a ABA set. But, remember, these F3's came to the SLSF as ABBA sets. The Alco FA A&B's came to the SLSF as ABA sets. Just FYI. Ken ps - Sure wish they would do a F7 ABBA set, with an extra F9B as an add on. All in b/y of course.