The Surface 3 uses a Micro USB port for charging. The official Surface 3 dock looks like it makes use of the full-sized USB 3.0 port and the video out port to get its job done.
The Surface Pro line uses a proprietary hybrid magnetic dock connector/AC adapter port (which was revised on the SP3), meaning two different dock versions exist (one for the SP1/SP2, and another for the SP3), and neither dock version will work for the non-pro line. I think the USB3.0 port and video out port are left free for the SP3 when using its dock; possibly the same for the SP & SP2.
For "docking" capability using 3rd party docks, the singular full-sized USB3.0 port would probably suffice given the switch from ARM to x86-64 (and the subsequent increased driver support). Not quite as sleek as the official dock, but a similar principle.