multiple viewports work like this:
3 viewports (left/front/right) each have specified FoV Lets say this FoV is set to 60°.
The center viewport is set to view 30° left and right from the center.
The left viewport is set to view next 60° to the left from the center viewport
The right viewport is set to view next 60° to the right from the center viewport
3 viewport in total will then give 60°+60°+60° = 180° of FoV
Now if your actual FoV doesn't match in game FoV for each monitor, it'll look weird on the sides. Downside to having natural horizontal FoV is loss of vertical FoV (as monitor in front of you doesn't covert lot of vertical)
PS: you can have as many viewports as you want, game allows it (and I have tested with something like 9 viewports), you just have to calculate correct FoV and adjust to fit in game.