It's a simple calculation. Just sum the horizontal resolutions and apply that in the video settings of the game. The vertical settings will remain the same.
So if you have three 1920x1080 monitors in horizontal position, it'd be 3x1920=5760 for horizontal and 1080 for vertical, i.e. 5760x1080. Notice that using three monitors will triple the load for your video card! You'd need something like an Nvidia GTX970/1060/2060 or the AMD equivalent at the least, for more eye candy a faster one.
If you can pivot the monitors, i.e. put them on the side, the calculation would be 3x1080=3240x1920.