Voltage divider for Xbee's ADC - what resistor values?
According to the product details, the output impedance is 100k&Omega;, so 100&Omega; would almost completely short the signal.
To get 1.2V from 3V, you need a ratio of 1.2 / 3 = 0.4  

Assuming a steady 100k&Omega; output impedance, you will need a single 66.6k&Omega; resistor from the sensors output to ground to divide the range down as necessary:

3V * (66.6 / 166.6) = 1.2V

Looking at the xBee datasheet, the ADC input impedance is only 10k&Omega;, so an signal impedance of ~40k&Omega; (which is what you would have with the above) is not ideal, and you may lose accuracy.    

Ideally, the best thing would probably be to use an simple non-inverting opamp placed in between this and the xBee ADC input to buffer the signal.