Hello,
If the sensors you refer to are the channel outputs from a RC receiver (as described in another post), a satisfactory interface method may be to simply provide a series resistor (say 4k7) for each input. The intrinsic protection diodes will limit the voltage swing. Alternatively, you could add an external Schottky diode between each input (anode) and +3V3 (cathode). The external diode should conduct before the intrinsic diode does. In both cases, the series resistor will limit current flow.
A related method would be to provide a voltage divider to provide 3V3 output for an input of 5V.
For any outputs from the MCU, you should first examine whether the external device will operate correctly with a 3V3 swing.
Regards,
Mac