All these bundles are based upon the Behringer Go Eurorack Case. The 15 and the 35, have one Go case, the 55 has two. The Go is 2 x 140 HP, with power built in for $300 USD. Hence the configuration.
I haven’t seen where someone has priced out the parts, to see if and how much Behringer may be discounting the bundle. Perhaps it’s worth taking this bundle and repurposing the Go case (or maybe raiding the power supply and connectors from it), and building a three high case as you describe Scot. It would be fun to see an image like from modulargrid. Perhaps there is a ready made case that fits this description.
If not, and people don’t do this for themselves there’s an opportunity for someone else. A few small Mom and Pops make their business creating accessories for Behringer synth equipment, and selling it on places like Reverb and Etsy. (They move fast to not get stepped on by the elephant.) Seems to me this could be a ready made business idea.