In our opinion, there can be only one "buttons window" per monitor. In other words, a third "buttons window" without a third monitor does not make sence.
So, if there could be a third "buttons window", would you add a third monitor in your equipment ?
I current use a single monitor with both existing pages. I would continue to use a single monitor and just position a third page where it could be easily grabbed for use. I see no need for trying to drive a third display.