When multiple instances of ST3 are open in Windows 10, the Windows task bar shows the current or active instance very slightly highlighted. When I say "very slightly" I mean very, very slightly--so slightly that it's almost impossible to notice.
I request that the active instance be highlighted on the taskbar more obviously, either by giving it a different hue, or making it much brighter; or making the non-active instances much dimmer; or changing the image inside the active or non-active instances, etc.
It should be instantly obvious which is the active instance.
A footnote to the above: Please test under varying brightness levels. I like to work with my computer set to a low brightness level. Less eyestrain.