Responding to Barb so everyone else will hopefully see this.
On the Samsung site, it's spelled out that Mirroring may not work if the screen resolution is not 1024x768. It doesn't exactly word it that way but if you change the Surface Pro 3 screen resolution to 1024x768 and then try to mirror, you will be pleasantly surprised. Didn't test DLNA connection but wouldn't work at 2160x1440.
What Microsoft should do is update the Video software to remember that screen 2, in this case the mirror, needs to be 1024x768. It's not, it's trying to project 2160x1440.
I hope this helps some, it solved it for me. But what a pain to have to make the resolution change!