User Sync

Hoping someone can advise on the best way to go about the following:

3 wordpress multisites. I was looking to use the user sync plugin to allow users of the site (multisite 1) as a master, then sync the users to (multisite 2) and (multisite 3).

Is this possible? Currently, none of the subsites will properly sync giving a sync error.

Any pointers would be helpful, especially if the plugin can actually do this using multiple multi-sites?