The only plugins that can be installed on the mixer are the following selection,
Link: https://shop.presonus.com/software/Software-Line--StudioLive-Series-III-Mixers/Software-Category--StudioLive-Series-III-Plug-ins
You must also have you mixer registered to your my.presonus account.
Your computer must have an internet connection to install the plugins.
Once you have confirmed this then please see the following articles that will help you install the plugins.
Link: https://support.presonus.com/hc/en-us/sections/360000942372-StudioLive-Series-III-Fat-Channel-Plugins