Yeah that's what I mean - when I went into Screen Resolution or whatever I saw my 2 monitors then the third.
When I clicked on the third one and chose to extend the desktop on it it turned off my secondary monitor.
I wish I could help more, but I sadly have only worked with 1 monitor at most. But, I did some research and here are some possible causes:
1) Faulty cable/adapter
2) Driver issue
Try to check if your drivers are updated, then try using the cable with a different monitor.