'pyrcc5' is not recognized as an internal or external command

Try running a batch file with the following:

@echo off
call "C:\Program Files\QGIS 3.0\bin\o4w_env.bat"
call "C:\Program Files\QGIS 3.0\bin\qt5_env.bat"
call "C:\Program Files\QGIS 3.0\bin\py3_env.bat"

@echo on
pyrcc5 -o resources.py resources.qrc

either place the batch file in your directory with resources.py or type in an absolute path.

For an additional information for people using ubuntu you can install libs like this

sudo apt-get update
sudo apt-get install pyqt5-dev-tools