Charge iPad Pro 12.9 on an USB 3 port? I just read through Fast Charging iPad Pro 12.9 and am intrigued.

However I'd like to use the USB 3 port on my MacBook air instead of buying a new power brick just for the iPad Pro.

Has anybody tried to use an USB 3 -> USB C adaptor in conjunction with Apples USB C to Lightning cable?

I haven't found an USB 3 to USB C adaptor only USB C to USB 3. Maybe that answers my question already.
I guess that's the answer. 
Wikipedia:

Adapters and cables with a Type-C receptacle are not allowed.(41)