Before you even begin to do something like this, it might be wise to look into how to accomplish it, before even making your video files. I've made videos to span up to nine 16x9 portrait monitors by using software and hardware to specifically do that. The video was made at a fraction of the size and the software/hardware scaled it up to fit.
With 3 monitors, it might be possible to just use VLC to do this, but I've never done it that way. There is an option for rows/columns in VLC, but you'll need to investigate that.
This is the wrong forum to be looking in. But I see you already found the correct forum to ask. The Live & Stage Events.