I have quite a few third party apps, and I was wondering if as long as my computer sees the repository, will it update from update manager? Thanks!
Short answer: yes it does provided we had added the third party repository to our sources, and updates are made available from there.
From the "Settings..." menu in Update Manager we can define how often we need it to check for updates available. Make sure to not choose "Never" to get notified if any newer versions are available for your release.
By doing so we get regular updates, e.g. for Virtual Box, or applications loaded from the Medibuntu, or the getdeb repositories. In case a third party repository is down or we do not want an update notification we may temporarily disable this repository by unticking it in the "Other Software" tab.