voltronic, the frequency/wavelength/phase interaction is a crucial perspective. I have a day job that I have to go to now, and I suspect that by the time I log in tonight, someone will have nicely filled in the basics. It's a perspective that everyone needs to consider when thinking about microphone placement, and specifically about mixing the signals from more than one microphone together under any circumstances.
Just to get the ball rolling, though: With some audio software or test equipment, you can record an audio frequency "sweep tone"--a simple tone that starts at one end of the audio frequency spectrum (say, 20 Hz) and slowly rises in pitch/frequency until it gets to the other end of things (say, 20 kHz), with a constant amplitude. This is a very useful signal. If you record it as a two-channel wave file and play it back, the tones that you hear should sound as if they're coming from a point between your loudspeakers. You could then take a copy of that two-channel signal, mix the two channels 50/50 into a mono signal, play it back again through the same two loudspeakers, and the effect would be indistinguishable (no surprise since each speaker would be playing back exactly what it had played back before).
Now if you go back to the two-channel sweep tone and take one of its channels and shift it in time by, say, 2 milliseconds in either direction, you can play that signal back and the mono effect will remain at low frequencies. But something weird will start to happen in the midrange and above. I want to keep this completely non-mathematical for now, so let me just say that at some point the clear center placement will dissipate and dissolve, then return as the tone goes higher still--and then it will dissolve again and return again, more and more rapidly as the frequency continues to rise.
Mix _that_ two-channel signal into mono (again 50/50) and instead of the weird phasiness, you'll get a centered-seeming tone with an amplitude that rides up and down quite severely. Wherever it was "phasey" sounding in the two-channel version, it will be lower in amplitude in this mixed version (even disappearing completely for a moment), and where it was well centered in the two-channel version, it will temporarily be several dB louder in the mixed version--so-called "destructive" and "constructive" forms of interference respectively. As before, the alternation between rising and falling signal strength will become more and more rapid as the frequency rises.
That (for reasons we can get into) is called a "comb filter" effect. And it is basically what you're doing to your signal any time you mix two inputs together that contain substantially the same material, where one is time-delayed relative to the other--including when the source is speech or music rather than a test tone. The sweep tone simply makes the point more obvious, since it isolates the effect on one frequency at a time. The problem with many multi-mike setups is that the differing distances from the sound sources to the various microphones cause time delays of this kind.
In professional recording situations, where dozens of microphones may be mixed down to two channels (or even just one channel for P.A. at live events), the effect gets kind of swamped by the sheer number of different inputs--and of course they aren't all picking up the same material for the most part, especially if the 3-to-1 rule is being followed. In a pragmatic sense you can record with a pair of microphones, or carefully blend in a few spot mikes, OR go all the way to the other side and use 50 microphones with or without a main pair--but where the comb filtering problem really gets you the worst is when you add the signals from just one or two additional microphones to the signals from a main pair, unless you're very careful about what those additional microphones are picking up. If they're coincident with the main pair, there are no arrival-time differences so this problem doesn't occur. If they're far from the main pair, the signals are so dissimilar that there's a random relationship between them and again, the comb filtering won't generally be obvious although technically it will exist to some small extent.
Gotta go. See y'all later.
--best regards