Well it does not exist yet.
https
/www.youtube.com/watch?v=0YzkpITCiUY They are working on a MXM 3.0 version. 2.0 x16 link will be fine for 3.0 cards as they are backward compatible.
I have tested an exp gdc and it works but with caveats, My test laptop, a Dell N7110, is sandy bridge i5 2410m with a geforce 525m. I have an exp gdc with a 750 TI and a dell DA-2 power supply. It takes an act of congress to make this run with the whitelist for wifi cards stopping it from booting. Got to figure out how to edit the bios to remove whitelists to make this easier. I believe the expresscard would be the better option for connection but not many laptops have this. Will try on a lenovo with an i5-560m and intel IGP when my expresscard cable comes in.
Remove the wifi card, install the cable, do not connect to the dock, boot the laptop, once on the desktop, close the lid to enter sleep mode, then connect the cable to the dock, open the lid and it powers up, Then the LAPTOP drivers from invidia are installed, and remove the 525m from device manager to free up resources. The card works on the internal display. There is a penalty from x1 link width and using the internal LCD, but it was still a huge performance boost over 525m. Don't forget a USB wireless adaptor since the wifi card is removed and don't connect it to the exp gdc.
This is all a very inelegant solution. The goal is to have the portability and function of a laptop, then when you need more power connect to a dock. USB-C and Thunderbolt 3 should make this better, think alienware graphics amp without the proprietary connector. Purchase new laptops with Thunderbolt 3 over USB-C and intel IGP only, then add the nvidia card and use the laptop drivers for optimus displaying back to the internal LCD for semi portable.