Changing the resolution works. The link isn't specifically referring to mirroring but it's why I tried changing the resolution. Somewhere else I read that it's the responsibility of the device, not the TV, to negotiate the connection. Perhaps the Surface Pro 3 isn't following the Miracast protocol. That would be a bummer, I was hoping I purchased state of the art.
Interesting. That page is actually specifically for connecting using a 15 pin VGA adapter. And it is interesting that 1024x768 works. Miracast is badly broken in the SP3. customers are upset and SHOULD be upset. Microsoft did NOT submit the SP3 for Miracast certification. So with two uncertified devices, it's an adventure, to say the least.