I currently have a website with its own user authentication. It also has 3 separate installations of WordPress in its subdirectories, each loading data from 3 separate databases. What I would like to do is load wp-includes/pluggable.php when authenticating users on the main site in order to carry out WordPress authentication at the same time, as outlined in the following answer:
However, as I have 3 WordPress installations that I would have to authenticate users for and generate cookies, this would be extremely difficult. Does anyone have any pointers/ideas on how I would do this?