Do you think three layers of 6 oz in strong enough? What is the recommended lay up?

I think it depends on the style of seat.
I use an NDK Romany - style seat which has a sloping back (no backband needed if you don't slouch).

There can be quite a bit of pressure on the seat back in that style, so I think I use a lot heavier layup than absolutely necessary.
I can't recall the layup I used last time, but it was probably more like 4oz,carbon,4 oz,6oz,6oz,6oz with added reinforcements at the 'corners' or where I guessed that there would be more stress. I've also used a layer of 10 oz 'boat cloth' in some seat and coaming layups, but it doesn't conform as well to sharp curves, compared to 4 or 6 oz cloth.
You can always do a light layup and then add more layers later if you are using epoxy. Three layers of 6oz is still pretty flexible.
BTW, I glass the seat to the sides of the hull.