I've created 3 lists .. and I've generated a subscription formular for one of the lists where you can optionally subscribe to the other lists.
The big big problem is that the custom fields are only filled on the primary maillist. Why is that? Can I change this?
This is actually a limitation of the multiple list signup - custom fields only go into the primary list. In many cases of course the separate lists don't even have the same custom fields, so it is not usually an issue.
If you do need the exact match, adding to all the lists via the API would be the comprehensive solution. You'd need some web development skills available to you of course.