You could back up the users table from site 1 and restore them to site 2 and 3 using a cronjob. But problem is the user table also contains information like number of posts, userlevel and other personal settings.
I installed x-soap to the first homepage. But i don't understand how it works. I cliked on the tab named "Configured Fields". In the drop down list I selected "User".
I can now see all tables and I can select checkboxes. I tried to check "Retrive" on some fields then post. I don't know if it was the right way. The only thing I saw was that I message appered "Databas updated". If I click on the "Configure plugins" tab, then I get errors. Do I need to install a torrent module to?