Front page meta tags and multisite configuration
It depends on how your sites are structured. Are all three basically the same site only available via different URLs?
First off, be aware that this can penalize you for having the same content available via multiple URLs, search engines don't like that. You should use Nodewords' ability to hardcode a specific base URL to pick one of the domains to say that one is the primary domain; the others are secondary that will help avoid penalties.

That, of course, is if you're using Drupal 6; for Drupal 7 there are other options.

Disclaimer: I am the maintainer of the Nodewords module.