I now have my setup working. I had to connect the Cisco DeskPro to the laptop HDMI port. I had previously tried to connect the Dell monitor that was flickering to the laptop HDMI with no change but had success with both Dell monitors off the dock and Cisco Desk Pro off the laptop HDMI.  Thank you for the suggestions. 
I have a 5500 (Windows 10) connected to a wd19 dock over USB-C. There are 3 monitors plugged into the dock: 1 monitor 1920x1080 with HDMI (DP Adapter) 1 monitor 1920x1080 with HDMI 1 monitor 1920x1080 with HDMI (DP Adapter) I can get two external monitors with the laptop screen (3 total) to run at 1920x1080, but when I plug in the third external monitor it flickers bad. I have updated firmware and Bios on laptop and dock. I have tried all configurations of monitors to ports and cables. I have tried to plug the third monitor into the HDMI port on the laptop. All have produced the same bad flicker on the third external monitor when connected. 
Precision 5500 wd19 dock and triple monitors, one monitor flicker