To expand on this, you can easily do something like:

xranrdr -d :0 --output VGA-2 --right-of VGA-1

xranrdr -d :0 --output VGA-3 --right-of VGA-2

That would line up your three monitors in a row. I've been using it to automate testing of a multi-window Electron app and it works very well. You can do grids, columns, a cross - whatever...