Cancel A Specific Reminder for App Store Update
Mavericks is free and doesn't cost anything to update to.  If you are satisfied with staying at Mountain Lion you can ignore any update for server.app version 3 by opening terminal.app and running..

softwareupdate --list


to get the full name of the update and then run..

softwareupdate --ignore &lt;label&gt;


If you due end up running the free upgrade to Mavericks you can clear any updates you ignored by running..

softwareupdate --reset-ignored