If you use a PC and want to code AS3, you can choose between Flex Builder 2, which rocks, but costs, and FlashDevelop with its new AS3 support and my AS3 project templates or the FlashBuild plugin, which rocks almost as much as FB2, but is free.
But if you are on a Mac, you really have to roll your own environment. Steve Nelson, a fellow Bostonian and active member of BFPUG, has taken my FlashDevelop project templates and tweaked them to nicely on a Mac. As the templates were Ant-based and used the command line compiler, they weren’t heavily platform-dependent to begin with, but Steve did the extra work of removing the FlashDevelop specific stuff and whatever other tweaks were needed to get it running on OS X. So here’s the updated OS X AS3 template:
https://www.bit-101.com/flashdevelop/OSX_ProjectTemplate.zip
Note that I have not actually used this myself, and it hasn’t been heavily tested, but I know a lot of people are successfully using the pc versions, and I trust Steve did a good job converting it, so you should be good to go, or pretty much so.