Is this for gaming? If so you can set the FOV in many games but there are often limits to how wide you can go, which in some cases can be worked around with third party software.
I'm not sure what projector you have but let's say you have a 1920x1080 model and want to mimic a setup with three 16:9 monitors in landscape setup (48:9 total aspect ratio). To get the 48:9 ratio, you'd have to sacrifice 720 of your projector's vertical pixels and run at 1920x360. At 1920x360, your 120" projection would be 118" x 22" at 16 pixels per inch (PPI). If you wanted to get a more reasonable pixel density, you'd need to be using two more projectors.