Third Monitor Won't Detect When Disconnected from MacBook

I have a dock that connects two external ViewSonic monitors to my MacBook Pro. It all connects, but when I disconnect my laptop, or restart, only one monitor is detected. If I disconnect that working monitor, the other monitor gets detected and I reconnect the monitor that initially worked. All is fine. I could just disconnect and reconnect every time, but that gets a little annoying. Any idea why this is happening?