Powering an RFID Reader + ESP8266 + Servomotor
You could use a boost switching regulator to generate the 6 volts from the nominal 3.7 volts of the single battery: -



This one produces 5 volts but there are plenty of boosters that can be set precisely to the value you need AND produce higher output currents like this one: -



The resistor feeding the FB pin determines the output voltage. Or there's this one: -



It can produce 6 volts by adjusting the resistors feeding the FB pin.

You could then use a buck regulator fed from the 6 volt circuit to generate 3.3 volts.