Sorry, should have read it more carefully.
Quote from Cooler Master N300 site:
Supports high-end graphics cards up to 320mm / 12.6 inch, including NVIDIA GTX 690 & AMD HD 7990
So most of 3 fan 1080 should fit, just make sure that the one you chose is no longer than 12.6 inch. Right now a 980Ti with 3 fans can measure 12.01", for example:
http://pcpartpicker.com/part/asus-video-card-strixgtx980tidc3oc6gd5gaming
Edit:
Gigabyte Extrme is even shorter at 11.30"
http://pcpartpicker.com/part/gigabyte-video-card-gvn98txtremec6gd
So all in all - no problem