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I started my flash programming career in AS2 which was quite bad and I knew barley any of it SOOOO I've started learning AS3 and in the first month of learning it I have learnt more than I ever knew of AS2 after about 2 years.
I think its because its much more organised and there are less ways to do things meaning its easier to know what you're doing.
Also you have the capability of scripting externally and has been created with OOP standards involved. I think AS3 actionscript is a definite for any flash coder new and old.
There are a huge range of tutorials, books and essays on learning AS3 and its easy to pick up so give it a try!