A more probable cause of the twitching of the servos when two transmitters are near the receiver is that the AGC on the receiver is being overloaded or a poor AGC circuit rather than 3IM.  Most good receivers will not have this happen even with the two transmitters right next to the receiver being used.  It is more likely to happen when the transmitter that should be in command is more distant  from the receiver being effected and the interferring transmitter is very close to the receiver being effected.   The receiver interprets  the interfering transmitter  frequency as noise and cranks up the AGC to a point that it's selectivity is destroyed.