...
P3 supports the Equinox P2 provisioning system and the Eclipse P2 UI, a friendly GUI to install and uninstall the software from the desktop (RCP) Eclipse tool. It has been ported to run on the browser by using Eclipse RAP, a technology that allows to port Eclipse RCP applications to the web with minimal changes.
Click on the Admin Workbench for Jira link:
...
There are two options in the Help menu:
...
Install New Software…: to install new software only
About: to explore the current configuration and uninstall software
You can install software from Eclipse Repositories (AKA Eclipse Update Sites)
Keep in mind that not all Eclipse-based Software can run on P3 without any change, it requires some grade of compatibility in many cases.