Change it so they all use the same db but different prefixes (if you’re not alreasy using that).
Then adjust the wp-config.php to use the same user tables for all installs: http://codex.wordpress.org/Editing_wp-config.php#Custom_User_and_Usermeta_Tables
Don’t forget to make sure the safety hashes of all installs are the same. And perhaps it is useful to set the cookie domain too: http://codex.wordpress.org/Editing_wp-config.php#Set_Cookie_Domain or as in this answer: How to transition cookies from .subdomain.domain.com to .domain.com with minimal impact on users?
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