Without a plugin such as Zaxwerks 3D Warps, you can’t fold a layer. Even 3D Warps doesn’t do it sharply, in my experience.
So you need 3 layers: one for each (shorter) wall.
If you have a continuous movie (or something) on the long wall precomp, you can mask it to make a shorter wall, dupe the precomp twice, then move the mask on each of the dupes to show a different part of the wall. Then you position the walls where you want, lining them up.
BTW, sometimes 3D layers that meet at a corner look best in Alpha Add mode.