Register One Time, Login to Every Site

I have three different opencart site;
my main site "main.xxx.com"
second is "sub1.xxx.com"
third one is "sub2.xxx.com"

I don't use opencart's multistore function. Every site has its own folder and database, but I want to use main.xxx.com for user registration. If you click create account button on sub1 and sub2.xxx.com, user will go to the main.xxx.com. After registration, user can login to sub1-or- sub2.xxx.com with his main.xxx.com account.

I want to get user informations from main.xxx.com and write automatically his info to sub1 or sub2.xxx.com database, when he logins to sub1 or sub2.xxx.com. Every user will register on main.xxx.com, but if user logins only to sub1.xxx.com, his account created only that site.

Can you help me about this situation? How can I make this or who wants to be interested in this project ?

Thank you.

It's not something easy, and it's good to have SSL on your websites, then send the data over to the other website to have it registered too. Lots of functions to link up, but they need to share orders too. Best to turn them into multi stores.