3 Monitors in a row and I want the dock in the middle
Switch off 'Displays have separate Spaces' in System Prefs > Mission control.



Then set the middle monitor to be the primary in
System prefs > Displays > Arrangement
by dragging the menu bar to that monitor  



After the question edit -
I'm afraid if you want the Dock on the left edge there is no way to achieve this. The above solution only applies if you have the Dock at the bottom, otherwise it has nowhere to 'sit' &amp; will always go to the first 'edge' it can find, which in this case is the left side of the left monitor.

You could try looking at something like DragThing instead, which can be anchored anywhere. I actually anchor mine to the right of my left monitor on a 2-screen setup, which effectively puts it where the red bar is on this image.

Late Edit
DragThing, after 20 years of sterling service, will no longer be supported after Mojave &amp; is no longer available for purchase.