Check out the TextMate AS3 Bundle from chromaticrain. Just download the Bundle, and dump it in “/Users/YOURUSERNAME/Library/Application Support/TextMate/Bundles”. Don’t forget to set the language menu (at the bottom of the Textmate window) to “Actionscript3.”

If you open the Bundle editor you can drag all your favorite commands and code snippets from the old AS2 Bundle or make new ones. Click on the AS3 Language Bullet (The one with the big “L”) under the “Actionscript3″ folding menu and you can add your own custom class names. This will make TextMate color them like all the native AS3 classes.