Question: I have three different domains all on the same server and I want to run the code on all three domains from one source on the same server, but not sure the best way.

Here's what I have:
domain01.com
domain02.com
domain03.com

domain04.com/sourcecode

I want domain01-03 to run the code inside domain04.com/sourcecode so the user can go to their domain and not have to go to domain04.com to see their site. I want to keep all the code inside domain04.com because I don't want to have to put the code inside each domain every time I make a code change.

For whatever reason I can't get my head around the best way to do this -- and want to do it right.

Any advice?
Thanks!

Answer: 
All you need to do is create a mapping on the first three sites to the appropriate directory in the fourth site, eg map `/domain04` to `/full/path/to/domain04/sourcecode`, then refererence its CFML resources via /domain04 in CFC and include paths. The inference here is the code does need to be accessible via the file system for all sites concerned.

Note that if you also want to server non-CFML files via HTTP (eg: images, css, js), then you will also need a web server virtual directory along the same lines.

None of this requires a framework, it's standard CF / web server functionality.
Are you using a framework? One like ColdBox could make this trivial if your code is written modularly. (Disclaimer, I am affiliated with ColdBox) 

If not, it really depends on what the code is. CFCs can be mapped anywhere via ColdFusion mappings. Even .cfm files can be included as long as the file systems are visible. If you're wanting to basically have complete copy of a site in another web root without duplication, I would first consider using a shared source control repo and a build process that checks it out in the appropriate places, and secondly a good old, symlink will also work .