Single sign on with different multisite environments

I have 3 separate WordPress Multisite environments setup. They are all on the same server, but completely separate installs with their own cPanel accounts. I'd like to setup single sign on so when someone sign up or signs in from one site it will sign them up or sign them in on the other networks.

I've seen a few plugins and possible ways of doing this, but would need a bit more direction. Please advise. Thanks.