You would need to create an interface that would connect to all three applications. I wouldn't suggest directly manipulating the database tables unless you really know what your at. Considering you have the source code, it should be very doable.
Make a list of all the required and optional fields of all the registration and login forms and create a script to pass these variables into the registration and login scripts of each application. You may need to change the post variable names to suit each app but its not impossible. You will need to have some knowledge of a server side language.
“The best thing about a boolean is even if you are wrong, you are only off by a bit.”