Continuous power between multi-gang boxes with 3 way switches
Yes, that's fine.  From the box with the supply coming in, You can run a /4 cable between the boxes -- 


always-hot (e.g. Black)
neutral (must be white) - this works because all loads are on the same circuit
two travelers (red/blue, or red/white-red depending on if it's 12/4 or 12/2/2 cable).  


However in this configuration, the onward power to the lamp must come from the other box.  

If you want supply and lamp coming into the same box, you would need /5, and at that point, for availability reasons, you are better off using /3 and /2.  The /3 would be entirely dedicated to the spur 3-way, and its white wire would not be neutral.  Onward hot and neutral would be in the /2 cable. 

As always, you are better off using colored tape to mark the wires by their purpose instead of being stuck with the default colors of cable, which are to distinguish wires.  The two travelers don't even need to be distinguished, I mark both with yellow tape. 

In the /4 or /2/2 configuration, you cannot substitute two /2 cables for the /4.  You will get eddy currents and vibration between the cables if you do: fatigued wires, arcing, trouble.  The wires in a cable are bound tight together for a reason.