Ubuntu (and its derivatives like Linux Mint) is a fixed-release distribution. This means it makes a snapshot of all the upstream Debian packages, and bundles them into a release. Those packages then remain at their current versions, unless there are significant security bugs.
Since OpenLP doesn't really encounter these security bugs, the version number remains the same because we don't package it directly for Ubuntu, and even if we did, we would not be able to get an updated like 3.0 into the already released repositories.
What a number of our users do is to download the Debian package and install it. This mostly works, but you may encounter issues as Debian packages are not 100% compatible with Ubuntu or Linux Mint.