SIERRA : How to activate a 3rd screen monitor on GTX 960 ?

Hello,

I upgrade my Hack. My new config :

- Gigabyte UD5 TH-Z170X
- Intel i7 6700K
- RAM 16GB Corsair LP 3000MHz
- Noctua NH-D15
- Gigabyte GTX 960 OC 2GB 1279Mhz
- I kept my 1TB SSD, HDD and my my Burn DVD from my old Hack with Fractal Design R3 + Corsair AX-650W.

Following your tutorials, I managed to install Sierra and Nvidia drivers web. I'm on iMac 17.2.

Everything works except my third screen.


QUESTIONS :

1- In "about this mac" my 960 is well recognized but with 0 MB RAM...

SIERRA : How to activate a 3rd screen monitor on GTX 960 ?