ORIGINAL: dirtybird
Bax I don't thinki your explaination is correct. It would not make any difference how close the transmitters are if that is the case. The mixing occurs in the receiver.
Distance does make a difference because the interfering signal is very much weaker than the intended signal. However, if the receiver's far enough away from its own transmitter, and the interfering transmitters are close enough, then a hit can happen. With the way almost ALL R/C activities are practiced, though, 3IM is a rather rare event. Modern equipment is very good.