The nRF51822 data sheet table 20, states VDD operational ranges 1.8V..3.6V, so the Pi 3.3V pin is voltage compatible.
The Raspberry Pi power data sheet, gives some guidance on calculating power budget, and therefore working out how much current might be available on the 3V3 pin on the GPIO connector
The micro:bit is specified by the BBC in their safety guide as 'do not draw more than 100mA from the edge connector'. So providing your existing configuration of your Raspberry Pi has enough headroom to supply up to 100mA from the 3V3 pin, it should be fine.
If you are connecting peripherals to the micro:bit with higher current demands, then you should design your circuit to power those peripherals externally.
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