Flowmotion,
You raise a fair question, how come all three beams aren't totally unique? Without getting to deep into multi-beam theory I think it comes down to the two main components i.e timing and returned target strength. Timing of the return is the first part, if a target is off to the port side the return comes back to the left side of the transducer first. But it just doesn't stop there it continues on to the transducers right side and beyond. Now the sounder tries to determine the time difference between when the target return was detected on the left side of the transducer vs when it was detected on the right side. It also is comparing the strength of the return. The greater the distance between a transducers elements the longer the time delay. In high end survey multi-beams the receiver array can be four to eight feet across, the elements the DFF3D transducer are less than an inch apart. The results is a very short time delay and the strength of the returns being almost equal.
Also a lot has to do width and angles that the beams are set at. Below is a pretty nice example of a few predator's working a small bait school off to the port side.
Snips