AS3 is the successor to AS2 (obviously =p).
AS2 (and AS1) were never really meant to be a programming language, just a light scripting language for helping do dynamic things in animation. They were really never intended for games. They are missing a lot of features and constructs that belong to a normal programming language and have slightly unusual syntax.
AS3 is the reworking of AS2, but it's been built into a rather powerful and much more feature complete programming language, with various advanced features like classes, inheritance, strict data typing, etc.