If the monitor is turned OFF.. the OS doesnt see it like unplugging a USB device..
if theres one less monitor it configures itself to use the only AVAILABLE monitors and sets its contents to be shown on them.
Perhaps if you go to settings and display and IDENTIFY the monitors with numbers that show up on the screen.. you can see which is MAIN DISPLAY (1) and which is second display which can be safely turned off (2) and which is 3rd display.. and plug them in, in that order or assign them in the display settings.. so whenever you wanna turn off a monitor you dont turn off the MAIN display.. you turn off the LAST (3) 3rd display.. uhh.. yeah sorry other than that i got nothing.
there really is an option to set a main active display though thats probably all you'd need really.