Thirds allow you to split your screen into three columns and/or rows. This is done via SizeUp's powerful AppleScript feature and a third-party application called FastScripts which allows you to bind shortcuts to run AppleScripts.
Start by reviewing the SizeUp, AppleScript and FastScripts document for background on how to setup FastScripts.
SizeUp Thirds AppleScripts
Size It Up
You can access the SizeUp Thirds Actions via the FastScripts menu bar item, or you can bind shortcuts to individual scripts in the FastScripts preferences.