I too have not been able to mirror to a Panasonic TC-65AX800U using Miracast with my SP3. I can, however, mirror to this TV using my Surface 2 and an Asus Laptop each running Win 8.1.
I have reset my SP3 to out of box condition and tried to mirror before applying any updates. No dice. I fully updated and still no dice. Clearly the Panasonic supports Miracast as evidenced by two of my three devices connecting.
MS, please acknowledge that this is a known issue the SP3 or I am going to return it.