have 3 CMS read from one plugin Folder?

[ohspublishing]

So I use this CMS on 3 different subdirectories on my site.

If I add a plugin I have to add them to each plugin folder. Is there a way to put the plugin in my main directory and point the subdirectory to the same plugin folder.

How and where in the code is the plugin folder link so I can change it to point somewhere else?

I use version 3.1.2 - all I want to do is change them so they read the plugins folder in a different directory. Hope you can help - let me know if you have a question what I am needing.