<p>I have an application built in Flex Builder 3.  It has a fair amount of mxml and as3 code which uses some other custom compenents.  I have looked at the documentation on building components which shows how to make a simple mxml or action script component that extends something like a combobox, but I'm lost as to how to take a whole existing and independently functioning Application and turn it into a reusable component. </p>

<p>Basically, I'd just like to create multiple instances of this app inside of another flex project.</p>

<p>Anyone able to provide a little guidance?</p>

If you simply want some "parent" Flex application to embed several instances of this autonomous child application, then you should see Adobe's
"Embedding Asset Types"
documentation, which describes how to embed one SWF file in another.
From the documentation:
You typically embed a Flex application
  when you do not require the embedding
  application to interact with the
  embedded application. If the embedding
  application requires interactivity
  with the embedded application, you
  might consider implementing it as a
  custom component, rather than as a
  separate application.
If you
do
require interaction between the embedded application and the parent application, you can look into the
SWFLoader
control.