For the Neuron 3120: The 3.3V Neuron has 5V compatible COMM and 5V tolerant I/Os, so the solution will be to use the 3.3V Neuron with a separate 3.3V voltage regulator for power supply. The rest of the board including the COMM transceiver can remain at 5V.
For the Neuron 3150: This Neuron chip has an external address and data bus. The 3.3V 3150 Neuron will require that the external memory accessed by the Neuron be 3.3V. Also there will be a need for separate 3.3V regulator to power the 3.3V Memory and the 3.3V 3150 Neuron chip. As with the 3120, the rest of the board including the COMM transceiver can remain at 5V.
Note: When using the 3.3V Neuron with a 5V COMM transceiver, pin CVDD has to be connected to a 5V power supply.