Need help with an AABBB arrangement, please.

[nimbleswitch]

While we're on the subject of the numbered Endings, the phrase, e.g., "D.S. al 3rd End." won't fit in a single 4-bars-to-a-line (system) measure, but I've found that you don't need that entire phrase. Just "D.S. al 3rd" works just as well on playback and will fit in one normal measure. Caution, though, as others have pointed out, there's no guarantee this will work in future updates or versions.