You are likely describing the "fish-eye" effect. Stuff on the middle monitor looks normal, stuff on the sides gets stretched-out (kinda zoomed-in too) the further to the right or left you are looking. It's normal to see that effect. What you should try is a program like Flawless Widescreen. FW has a bunch of different game plugins that can help make menus appear more natural and some games you can use a slider to make adjustments to the FoV depending on your own tastes. FW will also fix AC4: Black Flag so you can play it properly. There is also a program called Widescreen Fixer. Give those a try.
Flawless Widescreen: https://www.flawlesswidescreen.org/
Widescreen Fixer: https://www.widescreenfixer.org/
Also the widescreen gaming forum can be very helpful also: http://www.wsgf.org/
And yeah, as OriginalCadaver mentioned, make sure the game's resolution is set properly to your eyefinity resolution.