Threads in ActionScript 3

ActionScript 3 is a single-thread execution model; therefore, it doesn’t support multi-thread programming technique.

However, there are some tricks for asynchronous computation below.
Threads in ActionScript 3
Asynchronous ActionScript Execution
CJ's Blog: Asynchronous Computation

Mutexes in ActionScript and JavaScript

There is also a third-party thread library for Actionscript 3.
http://www.libspark.org/htdocs/as3/thread/