Thread:Project:Support desk/Need help setting up unified login process/reply

I don't know of a tool, which could use only one DB for those three systems. So the most important question is: Which system do the original user accounts come from?

If you need to know how the login process for MediaWiki works, you could use the maintenance script createAndPromote.php as a first starting point. It contains code, which creates a new user account in the MediaWiki DB (and makes it an admin, but this part obviously is not relevant for you).

You said something about intercepting the login process: Yes, that's possible. MediaWiki has many hooks and some of them are called during login. There you can get and manipulate the login data.