using multiple monitors with MacBook Air

I'd like to use my macbook air with two other monitors. I have both monitors plugged into the dock with hdmi cables, and the macbook air connected with the mini display port, but I can only see one of the monitors in the display/arrangement options. How do I make it so that I can use all three monitors (the two external monitors and the macbook air monitor)? I'd like to view different apps on different monitors. tx