I tried to run Crysis 3 on three Monitors (5760x1080) with two HD7970 GHz but I can't change the HUD so that it fits on the Center Monitor.
Is it even possible to do this?
Also the red dot of the gun sight is stretched.

I don't know about centering the HUD, but I'm pretty sure you can fix the stretching on your sight by adjusting your FOV. Which, if memory serves, there are cVARs for, posted somewhere in this forum. Maybe SDub or one of the other vets can dig it up for you?

I can change the position of the HUD in the Game Options but not enough to fit it on my Center Monitor. I tried different FOV settings but the HUD position doesn't change.

http://youtu.be/DatSjuIu0DY

In order for the HUD to be located on the center monitor, it would have to be coded that way. I know the guys over at Widescreen have done some amazing stuff for ultra widescreen, and I've used some, but give it awhile and I'm sure they will get a "widescreen patch" to make the HUD play nice on the center monitor. I've finished the game with the HUD where it is, and besides having to look left and right a few times, it's not so terrible. Given the fact that the entire game plays in HOR+ is a testament to Crytek partnering with AMD and supporting ultra wide resolutions. However, it can't be that hard to change a few lines of code. To support ultra wide resolution, games like the STALKER series and SKYRIM have had some patches or mods released that allow the HUD to be centered on the main middle screen. Given that Crytek partnered with AMD on this trip, I find it kind of hard to believe that the game doesn't already support it to be honest. However, FMV scenes in C3 are also squished, unless, of course, you're running a 6 screen set up.

I've got a three monitor setup as well but can't get a 5760x1080 resolution option to come up. What is the "magic" to get the three display working with Crysis 3?