Do you have System Preferences -> Mission Control -> [X] Displays have separate Spaces enabled?
You should be able to drag your mouse off the bottom edge of a screen where there is no monitor under it. It does not work if you drag off the bottom edge of a monitor that has another monitor directly under the cursor.
So in your picture, you need to drag your cursor off the bottom edge, of the upper center monitor, in that gap between bottom monitor and the left or right monitors to move the dock to the upper center monitor.
But the Dock is going to move to whatever monitor you drag the cursor off the bottom edge of a monitor into a void.
NOTE: I keep my dock on the left side of my 32" 4K monitor, and I'm with Luis in that I do not use my dock all that much throughout the day. Yes I use it, but it is just a few times a day, and I work from home on my Mac, so I'm spending 10-12 hours a day either working, or just surf'in the web and such.
Command-Tab is used a lot more than the dock. Or in my case I have my open windows arranged on my screens so that there is almost always a corner of an App's window visible on the screen, my windows are arranged is a specific pattern, so I know exactly where to click my mouse to bring up the app. I even use the utility "Moom" to reposition my windows to where I want them (there is also the utility Stay that does more or less the same thing).
Moom and Stay are also useful when you add or remove monitors because you disconnect your laptop and take it somewhere in that you can have profiles for different monitor setups and Moom or Stay can place and size your windows according to the current monitor setup as you change locations.