If you want to change the charger circuit, there are two ideas. One is to change the feedback resistance at the output end, and the other is to change the reference voltage of the chip (however, if it is a dedicated chip built-in reference, it can not be changed). Then there may be low-voltage protection, down to 3V will definitely protect the action, so you have to change the protection circuit. If it's not a switching power supply, it's pretty hard to change. Another idea is to add a step-down circuit at the output end of the charger, if the current is not large (within 1A, the simplest 78XX or LM317 can be done) with a linear power chip plus a step-down
Two types, some changeable, some unchangeable. Without TL431 three-terminal IC can be changed, with TL431 can not be changed. Because the 431 is 2.5V, the light-emitting diode of the optocoupler PC817 requires 1.1V, for a total of 3.6V. Therefore, the minimum power output voltage of the 431 integrated block is 3.6V. You don't need 431 to output 3V. Just adjust the component parameters a little bit.