If nothing is connected to the USB connector, battery, or 5V pin (no input voltage supplied to the LDO), it would be possible to supply 3.3V to the 3V3 pin. However, the 3.3V must be constant voltage.
If nothing is connected to the USB connector, battery, or 5V pin (no input voltage supplied to the LDO), it would be possible to supply 3.3V to the 3V3 pin. However, the 3.3V must be constant voltage.