I have no experience with triple monitor setups, and so my comments here must be taken as general.

For a single monitor, the FOV is measured from the left and right edges of the display.

For triple monitors, if the FOV is measured from the left and right edges of the whole combination (L edge of L screen to R edge of R screen), then this 'tank slit' view chops off a huge part of the view on both the the top and bottom.

To be more effectively utilized in an air combat sim, where sufficient vertical field angle is a must, the triple monitor FOV must be expanded very significantly. But without suffering the horrible gnomonic distortion introduced by the tangential projection necessary for correct polygon drawing.

Indeed, the limit to FOV in a single tangential projection is 180 degrees. But at this limit, the image scale variation between field center and edge approaches infinity! That's gnomonic distortion at the ultimate extreme. Utterly unusable.

And so the implementation of more than one screen should handle the drawing for each quite independantly, with each having its own center of projection. Then can you set really large FOV values for the whole combination. For instance, if the game treats the FOV value as applying to a single screen, and you set this as 90, the total horizontal FOV for three screens becomes 90 * 3 = 270 degrees. On the other hand, if the game expects the FOV value to apply to all three screens in combination and you set 90, the FOV of each is but 30.