I had a small AS3 project and decided I would stray from the standard Flash Builder solution provided by adobe to see how things have changed over the last few years. I ended up looking at two solutions. Being major TextMate fan I set it up and hooked it up to the Flex SDK. On the recommendation of a colleague I checked out the latest version of FDT as well.
TextMate 2 still supports AS3. Due to the broad nature of the application it does require some setting up. There is a plugin for As3. The plugin however is not the only part of the setup process. See setup instructions here. If you already use TextMate or have an affinity for it then its very usable once you get through the setup process.
FDT used to be a Eclipse plugin back in the day. Some where between FDT 3 and FDT 5 it became its own app. Well you get eclipse pre-bundled with it. FDT used to be quite pricy. How ever now they offer a free version. The free version does not have all the bells and whistles you get with the paid for version but if your looking for is a quick and easy way to compile AS3 projects. It has a two step setup process and practically works out of the box. The only thing you have to do is hook it up to the Flex sdk then all the settings have defaults and you set them all in a GUI.
In short FDT is the easiest and quickest solution if all you want is to get in, compile then out.