I bought an OptiPlex 9010 on the basis of http://www.dell.com/uk/business/p/optiplex-9010/pd which says "Configure your workspace the way you want it with support for up to three native monitors via DP/DP/VGA ports."

I have connected three monitors to it, but I can only enable two at a time. It can be both of the ones connected by DisplayPort, or one of the ones connected by DisplayPort and the one connected by VGA. How do I enable all three?