Do they have to be carbon fiber? Master Airscrew makes them in this size in both pusher and tractor versions; available immediately in unlimited quantities for about $10 each.

Xoar has them.  I didn't see weight, but the 2-blade electric is about 18 gms.

https://www.espritmodel.com/xoar-propellers-3blade-electric-pjie.aspx

The 12 x 6 was in stock.

$52 each!

They are also wood, not CF, but fairly light.  At that price, I didn't suggest he get the " handful" that he mentioned :-).  Light 3 blade for electric in 12 x6 are not plentiful from what I could tell.  I also looked a while back.  There may be a European 12 x 6 CF prop, but I could not find it again.

Here's another approach: buy three 12-6 wood props and make two 3-blade props out of them. Badass shows those being available in both rotation directions at $8.49 a prop, which means it will be about $12 per each 3-blade prop. I can help you with that if interested. I used to make 3-blade combustion props this way; they all held just fine.