How to make a switching source for voltage converters
What about this: -



It seems like it would do what you want (or at least my interpretation of your requirements).


When the supply is 3.3 volts, the output from the buck-boost (7.5 volts) powers the buck regulator.
When the supply is 6 volts, the buck will receive power from the 7.5 volts or, if the 7.5 rail is turned off it will receive power from the 6 volts rail.
When the supply is 8 volts, that 8 volts will supply the buck.