Can the current model MacBook Pro (2017) 15" power 3 Displays from one port?
It's a matter of available bandwidth of the Thunderbolt port, not the dock itself.

A Thunderbolt 3 port has enough display bandwidth to handle a single 5K @60H display or two 4K displays at 60HZ.  

On the 15" MacBook Pro (2017), it has two Thunderbolt controllers, so you can now attach a second 5K display or two more 4K displays.  But because it's on a second controller, it will have to be on a different port.

From Apple's Thunderbolt Page


  Thunderbolt 3 provides twice the display bandwidth of Thunderbolt 2,
  enabling iMac to support two 4K displays or one 5K display. And with
  two Thunderbolt controllers in the 15-inch MacBook Pro, you can send
  graphics to dual 5K displays for the perfect high-resolution photo and
  video editing setup.


The dock can support 2 displays, because that's the maximum that can be supported by the port.  To get a 3rd and 4th display, you would have to use a different Thunderbolt port on the MBP.

However, you could (theoretically - meaning I have never personally tested this) put a USB to DVI/DisplayPort/HDMI adapter on one of the USB ports of the dock and get a 3rd display, but I currently can't see how you would get 4K out of it.