Two ways: (1) add a discrete video card that has three video ports.  Dell RX640 has two mini DP + one DP port. (2) connect three monitors via Display Port daisy chain.  You would need DisplayPort MST-capable monitors for daisy chain.
Due to the work I'm doing I want to have 3 monitors, but i only have 2 display ports and then USB ports. Is it possible to run a 3rd monitor, if so how? 
Optiplex 3090, three monitors, two DP?