Is geeklog modular enough to be installed in such a way that three different sites (with separate public_html and admin folders) could be run off of a single install? I would like to be able to upgrade the core functionality for all three at once without losing any of the sites' customizations. Has anyone tried this? Thanks...

Depends on what exactly you want to share ... Each site will need its own config.php and lib-common.php (since that includes the config.php). After that, you can set the paths in config.php to share at least some parts, e.g. the system and the language directories. bye, Dirk