Sounds like supply 1 is your normal power, supply 2 is possibly a UPS or battery backup and supply 3 is a generator? What you seem to want is automatic load transfer switching. However, you don't provide much information?
You don't mention how supply 3 is started or the fuel it uses. Fuel isn't important but how it starts is important. You also don't mention the load current(s) or voltage(s)?
You need to better define things and start with a piece of blank paper and plan your logic in detail. Like how supply comes online? What you are looking at again is load transfer switching but much more information is needed including how you want to go about it?
Ron