When a 3.3V zener diode and a 5V zener diode are connected in parallel what is the average reverse break down voltage?

There will be no "average" reverse breakdown voltage. When the applied voltage reaches 3.3v, the 3.3v zener will conduct and the voltage will go no higher. The 5v zener will never conduct because the voltage across it will never go higher than 3.3v The only way the 5 volt zener will ever conduct is if the 3.3v zener fails. ALAN H.