Hi T0MR0, XSA,
I am using the plugin on Volumio3, it works as good as on Volumio2. It required the node modules to be re-compiled after installing the plugin, besides that no changes are required.
sudo apt -y install build-essential
cd ~
wget http://plugins.volumio.org/plugins/volumio/armhf/user_interface/rotaryencoder2/rotaryencoder2.zip
mkdir ./rotaryencoder2
miniunzip rotaryencoder2.zip -d ./rotaryencoder2
cd rotaryencoder2
volumio plugin install
## when installation is finalized, press CTRL+C ##
cd /data/plugins/user_interface/rotaryencoder2/
npm install --save onoff@6.0.0