From [100 watts OTL amplifier circuit using transistor MJ15003-MJ15004](https://www.eleccircuit.com/power-amplifier-otl-100w-by-transistor-mj15003mj15004-with-pcb/):
[![enter image description here](https://i.stack.imgur.com/GAM2J.jpg)](https://i.stack.imgur.com/GAM2J.jpg)

What is the purpose of C5 capacitor?

Why are there 3 diodes used? Not 4?