The Precision 5520 only has two video out ports =
USB Type-C Thunderbolt
So you need to look into getting a Dock that supports three monitors. Something like the TB16.
I also have the dell M5520 precision laptop and based on your reply, just want to know if I can drive two external monitors using the one HDMI and the one Thuderbolt port?
- If the answer is yes, can they be set so that they are additional screen spaces vs mirroring the laptop display?
All three at once, Laptop LCD and two external monitors, I don't think so. Perhaps lid closed, monitor on USB Type- C as Primary, monitor on HDMI as secondary? Which is why I referenced the TB16. You simply need to test it (Laptop LCD and two external monitors) and post the results for us.