You want to use more than one external monitor with your laptop the setting isn't accepted.
If you only have one external display port OR you want to use 3 monitors: the two external and the laptop - you must buy an external videocard. USB to VGA adapters
Disable or render moot the laptop display. I choose the option to duplicate monitor 1 & 2 - then to extend to monitor 3.
In line with solution 2, I closed my laptop after disabling the advanced power feature under the 'lid' category so that the laptop wouldn't go into sleep mode.
If your laptop supports a dock accessory you may be able to simply add a 3rd monitor and reduce the hassle of plugging in a bunch of stuff - but in my experience the 3rd monitor is usually of USB quality which even at top speed can show some lag.