One Thunderbolt, two 20" Cinema to an 11" MBA?

EDIT: yeah seems you can't

This guy tried with apple's own displays and cables and it only powered one

https://www.youtube.com/watch?v=_uUbchklSjY

I don't think you would be able to run 3 external monitors even with the Air in clamshell mode/some peripheral attached. The specs page for the current Air (here) allow the internal display + one external, and that should be the same for your 2012 model.

Apple has a KB article about the amount of (Thunderbolt) displays you can use: http://support.apple.com/kb/HT5219#18

If you use a Thunderbolt dock you can only use 1 display on the non-Thunderbolt port (usually hdmi or dvi), the other display has to be a Thunderbolt display or a non-Thunderbolt display hooked up to another Thunderbolt device (i.e. a second TB dock).

If you want to use 2 or more non-Thunderbolt displays things get complicated. Matrox has some options for you. Those devices will present the 2 or 3 displays you connect to it as 1 display to your Mac so OS X will only see 1 display. It can be extremely annoying, especially when using apps fullscreen.