Cannot connect third monitor to my laptop/dock station

Here is my hardware (for some reason only the S/N for the dock is working in the "Add Product" function:

3 x P2414Hb Dell Monitors

• 2 x connected from the two Display Ports on the dock to the display port input on the monitor. 
• 1 x connected via HDMI (dock) to Display Port (monitor) input on the monitor - this is the one that is not recognized by either laptop.  

1 x WD19DCS Dell Performance Dock 

1 x Dell Precision 7560 Laptop 

(I also have a Dell Inspiron 13 5310 laptop that recognize the third laptop either.)

I've also tried the following variations to eliminate any issues with the monitors or cables:

1. Two brand new HDMI to Display Port cables (neither worked).
2. Connecting each of the monitors to the display port on the dock (all three monitors work when they go direct to the display port).
3. Both laptops have a HDMI port on the back so I tried connecting to that port directly as the second monitor.

Nothing has worked. 

It seems like, from what I've read, this should be possible with my graphics card/GPU but maybe I'm wrong? Any suggestions/recommendations would be greatly appreciated!