Well of course... Even if you could display the game over three 16:9 displays, the FOV value would have to be set to... 156 only !
http://www.widescreengamingforum.com/fovcalc.php
Your resolution trick does not work : I added 1600x900 and replaced 1280x800 with 1280x720, but the game doesn't "see" these changes to the resolution list. By the way I can select 2560x1600 and play at that res no problem, although it is not listed in VideoOptions.lua... so it's probably not the right file (or the only file) to edit.
The Painmenu trick works though, it's great to be abe to apply a resolution change without restarting the game
