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.

I know this is an old thread but.....

The "extra" pair (blu w/wlu) is often used for DC voltage <=25VDC. I have no idea if its just a convention or a standard...but I always assume there could be applied voltage on the pair and terminate accordingly.

Since the intended "correct" use of a RJ45 is ethernet, make sure the actual use is well labeled.

The shared ground is more of a capacitance issue (bigger if you attach both grounds on one side w/brn + brn  and only one on the other side) and has been the subject of many a debate in the past….but my experience is that it never becomes an issue unless you are exceeding 100 meters)

just my thoughts....