Blocking current from Backup battery (3x1.5V=4.5V) when powerbank (5V) is turned on my timer switch
I have just the circuit for you, here: What does this MOSFET do in this voltage regulation circuit?
This circuit uses a P-MOSFET and a diode and does exactly that function, with a bonus of little or no voltage drop when the battery is powering the circuit.
Use the P-MOSFET side on the AA side, and a Schottky diode on the power bank side.