Actually, with
@Charlaquin's idea, B would finish his long rest half-way through C's 4-hour watch.
If you want a more even set-up, you could also do it like this:
View attachment 119814
White and Gray spaces are watches, Blue are sleep. For "long rest" your 8-hours are gray and blue, not white.
This way at dusk and dawn, the most active times IME, you have two people on watch for an hour at least. Also, C finishes his long rest first, so it is best to have a stronger character on that watch. Characters with the most resources remaining are good for A since they finish their rest last.