Off Relay3=Off, Relay2=Off, Relay1=Off
Low Relay3=Off, Relay2=Off, Relay1=On
Medium Relay3=Off, Relay2=On, Relay1=Off
High Relay3=On, Relay2=On, Relay1=Off
Power is only applied to one output at anytime. Just set the relays for the desired state. I would keep it simple and call a function for each state. Each function turns on/off the relays to match that state.