Motherboards for 3rd gen iPad Pro available?

Yes. Because Apple loves to screw over it’s customers.

But on a serious note, since it looks like this is a newer Apple product, supply is likely low for the parts. Also, Apple is likely doing as much as they can to restrict who has access to the boards since Apple believes that only they are worthy of doing repairs on their devices. Unfortunately this is just Apple’s way of sucking more money out of you after you have already handed over your hard earned money for what I believe is an overpriced tablet.