How to create a video so it can be displayed on 3 screens
I do this all the time at work, I'm currently rendering a 6 screen video wall for an installation. The easiest way to work is to create the video at the size of all the displays put together, e.g. for 3 × 4K displays it would be 11520 × 2160 pixels. This is so that you can work on the whole thing at once, and also so you can justify to your boss the insane amount of money they spent on your workstation (did I say that out loud?). 

You can use standard video tools like NLEs, e.g. Premiere or Resolve, compositors and animation packages like Fusion and After Effects or 3D applications like Maya, Lightwave or Blender. All modern versions of this kind of software will have the capability to work at custom resolutions, but as mentioned, your hardware will need to be pretty good, you'll have to budget for render time, and your proxy game will need to be strong (proxies are low-res substitutes that you can use to work on, then swap for the high-res originals at export time).

Then you export, and encode into the playback format suitable for your playback devices. Some playback devices are able to handle large videos and split the playback into multiple screens. Small computer-based devices like Intel NUCs and Mac minis can be hooked up to multiple monitors and standard video players like VLC can be used to play the content full-screen across them all. Alternatively you can get multiple playback devices, one for each screen and synch them over a network. The output format will depend on the hardware you are using to play the material, so talk to whoever is setting that up.

To split the composition you re-compose the full composition into the screens that will be displayed on each monitor. In the installation I am working on at the moment there are 6 screens, but the material is being played as three parts, with each playback device playing to two screens. So I've split my 10800 × 1920 into three 3600 × 1920 compositions.

The tools above all have the ability to take one composition and nest it into another at a different resolution, but as I've found in my current projects you may have to re-edit the main composition to help with renders (I was hitting a GPU memory wall with some GPU-based effects, so I had to resize some of my layers and shuffle things to get it to render without choking).