I’ve got a Surface Pro 4 running with a Surface Dock and 2 external monitors.
Looking at Advanced Display Settings
monitor 1 is the Surface Pro
monitor 2 is the Left external
monitor 3 is the Right external
is set to: Extend these displays
My problem is that I can drag apps and windows between monitor 1 and monitor 3.
But I cannot drag anything to monitor 2 (it just shows the standard Windows 10 background and nothing else).