Hey WendyMiller23! I connect 3 external monitors and continue to use the Surface Pro display almost every day!
I have a plug type dock that supports all 3 external monitors + the Surface display.
How I accomplish this is through 2 mini display to HDMI cords from my plug type dock and a Microsoft Wireless Display Adapter. All connect and work as extended desktops, no duplicates.
It's worked for my Surface Pro 4 i5 & my current model, Pro 5 i5, so I can't see why it wouldn't work for the Pro 7!
(Pictured below: All 4 screens working as extend. The other monitors only looks unnaturally large because of the high res of the Surface, with lower res monitors while showing the same window in ALL at the same time. This does not have this effect when a window is only displaying on one monitor at a time.)