Allows you to give a user upto three special ranks!
Clone into phpBB/ext/posey/multiranks:
git clone https://github.com/MrGoldy/multiranks.git phpBB/ext/posey/multiranks
Or download the zip and upload contents to phpBB/ext/posey/multiranks/
Go to "ACP" > "Customise" > "Extensions" and enable the "Multi Ranks" extension.