Maybe the fvwm man page section "COMMANDS AFFECTING WINDOW MOVEMENT AND PLACEMENT" might answer your question.
Using the -geometry
things known from X Window alone might do the trick.
Remember, in FVWM a screen is just a visible rectangle of a desktop.
So, if you want to place a window on the third column screen, on a computer with a 1920x1080 FHD display, just place it at x coordinate 3840 or higher.
My personal preferred configuration is 3 desktops each 4x4 screens.
BTW, with FVWM you are not limited to the physical screen size, as you can pan through the desktop (use the pager with middle mouse button). This can sometimes be very convenient.