[dabean]

Still simple if time consuming method is to create 4 databases (presuming you have a single mysql server).

With database 1 containing only the user related tables.
e.g. avatar, customavatar, privatemessage, profilefield, user, useractivation, userfield, usergroup, usertitle

Then the place all the other tables in the 3 databases.

Now your going to need to set the mysql user accounts with access to the user database and access to the forum database for THAT site.
e.g.
MySql Account: Site1
Database access
user,
forums1

MySql Account: Site2
Database access
user,
forums2
etc...

With this done go through all the code changing the reference to any of the tables in the user database to <database>.<tablename>