Using one user signup for multiple installations of wordpress on one server

Hello,
I was wondering if it is possible to use one user signup for multiple installations of wordpress on one server. I am using wordpress and have one installation for my main site in the root directory, then have two other installations on subdirectories for a blog and a different articles section as it was much better for organizing this way. For my users I would like to have one signup for everything if possible so users will be able to post comments on the entire site https://www.china232.com

One more question, if anyone knows any ideas of how i would then be able to use this one signup and integrate it with an smf forum so everything would be under one login and I could use the same user profile it would be much appreciated. I realize this could be quite a difficult question, but I just wondered if anyone had any suggestions.

Much Appreciated!!!