2x 1920 = 3840
-> 2 monitors = 3840x1080
3x 1920 = 5760
-> 3 monitors = 5760x1080
The heights remain the same (except if you place the monitors on top of each other), the width can be added together. Wasn't that hard, was it?
However most games probably won't support resolutions like that but for CAD software it will be quite nice.