Is it possible to use a Surface Pro 3 (or other tablet/laptop/netbook/desktop) and two Microsoft Wireless Display Adapter (or other similar devices) to extend a Windows 8.1 desktop (or other OS) so it uses all 3 screens (not just mirrored)?
In any case, can you provide information backing up your answer? I've seen contradicting answers on Amazon.
You can only connect to one Microsoft Wireless Display Adapter at once. When connecting to a second one you will be automatically disconnected from the first one. You can bypass this behaviour if you connect another Miracast sending dongle to your Surface, then you can connect to a second screen. In other words: You need one hardware capable of sending miracast signals per dongle.
No, your going to run into two problems.
Problem 1
Miracast only supports one connection at a time. https://actiontec.zendesk.com/hc/en-us/articles/205702149-Can-I-connect-several-Intel-WiDi-or-Miracast-devices-to-the-ScreenBeam-receiver-simultaneously-
Problem 2
Even if you could get around the one miracast adapter you may or may not get the full resolution on your displays. In my testing on three separate monitors (Acer H203H, HP w2007, and AOC 22) I was getting 1280x720 instead of the native resolution of the monitors (The AOC for example runs at 1920x1080 natively).
If you can I would recommend that you get a USB 3.0 hub and then pick up a couple of USB 3.0 to (your preferred display connector) HDMI for example and go that route.
