You should be able to use one of these parameters with the SCREEN() and SCREENVISIBLE functions :
- 2
- Back2
- -1
In fact, "-1" might be a really good choice, since it should always point to the screen to the immediately LEFT of the current FrontMost screen. But I have NOT tested this.
I don't have 3 screens to test, so I can't be for sure.
You can also look at this macro:
MACRO: Move & Resize Front Window to Othere Screen (Monitor) V2 Using BACK Screen [Example]