Use 5v Linear Regulator and LDO 3.3V
Use a non-linear power supply (i.e. a buck converter (like LM2596) in your case) for conversion from 12V to 5V. Then use a linear regulator (like AMS1117) to convert from 5V to 3.3V.


The reason why I have suggested using a non-linear power supply (for 12V to 5V) is because the heat generated by the linear one increases quite a lot with a high voltage difference and the current being consumed. This also means quite a lot of power loss. 
It is safe to use a linear power supply for 5V to 3.3V as the voltage differnece is less and the consumption of high current is not expected at logic levels. But if the 3.3V devices consume upwards of about 500ma than it is better to use a non-linear one  or a linear one with better efficiency. 
You can design the pcb such that the inductor and xbee radio are far away from each other or you can mount the inductor externally also. I don't think that a inductor of the size found on buck converters will cause a issue.  
Use the existing schematics of development boards like Arduino, ESP, xbee shields, STM etc. to get a better idea on how to deal with this issue.