Anyone provided three DMX signals over a single CAT5?

You'll find that most mutliport DMX products share the ground reference pins, so I don't see any issue with this.
There shouldn't be any issue with using the spare blue pair, so long as you remember it's a non-standard pinout and don't directly connect this to something that would care!

There is also a company that supplies adaptors to do just this, even achieving 4 universes with STP cable!: pub.tmb.com/.../SneakSnake-LTR-web.pdf

Thanks for the clarification. And thanks for the product reference.

Thanks for the clarification. And thanks for the product reference.