so many less rules to remember with baffled omnis!
:shrugs: ymmv
Amen! 
I dont use rules or templates anymore, I find that I always have to vary the established configurations to get the sound that I am searching for. I never use DIN, DIN(a) or XY. I always use A-B, Blumlein, M-S, 3 omnis, or modified NOS( 110-120 degrees and 12-14inches capsule separation),OSS, and soon, Decca Tree. 
Funny thing is that I find my desire to be discreet, the bands config, and the venue's layout and attitude towards me drive my setup more than anything else, LOL! God forbid, I could actually setup the way I'd really want to. Heh...
Truth to all that. Now we're in the real world again!
Honestly I was hesitant to even comment earlier in the thread, given my amateur status and with a web page of the inimitable MG stating otherwise. No doubt overly oppressive, control freak, freedom hating rule makers dreamed up the 3:1 rule eons ago. My
personal take is that it applies to individual mics on separate sources and doesn't apply to stereo mic’ing. Another personal take is most of these rules are really just guidelines and suggestions and nothing beats experimenting and listening. I’ve learned more from moving mics around while monitoring through some phones with good isolation than from anything else, just from the immediate feedback I get. But, two things irked me on to post:
1. simple practical application - 3:1 works and works really well for individual mic’ing of separate sources on stage or any sources summed together in my limited experience. 3:1 doesn't work so hot for spaced omni stereo arrays in my slightly less, but still limited experience (ignoring geometry for now). In fact, off the cuff, I’d say reversing the ratio to 1:3 isn’t even enough most of the time and 10:1 is probably more like it, but I’d certainly never state that as a rule. If the rule states that a spaced pair of mic’s in a stereo pair should be 3 times farther apart than they are from the source, that rule just plain doesn’t work
in my experience. Which makes me think it’s a mis-application of the original rule, after all aren’t these rules supposed to be good starting points at least? (especially since they’re only suggestions now that we’ve broken free from the domination of rule making oppressors)
2. some rules just can't be broken or agued away unless overwhelming evidence to the contrary is demonstrated – I’m talking about the laws of physics and mathematics. It remains a fact that two mics can't be 3 times as far away from each other as they are from the same point source, unless everyone in the venue uses their cell phones simultaneously to place calls to Steven Hawkings’ answering machine (which oddly enough sounds just like him). The rule just can’t apply since it is overruled by the pesky, domineering, micromanaging universe we live in.
The only time it could conceivably be applied to a stereo pair is when mic’ing a very large source (like a choir or orchestra that can be considered one source, guitarist stage left and keyboard guy stage right are two separate sources if your stereo mics are on stage) with the mics very far apart and very close in, at well a 3:1 ratio. Try it and see if it seems like a good rule. Seems to me there would be a rather distant and indistinct center image, but I’ll defer to those of you who get to do this sort of thing regularly.
Other arguments:
Arguing that it doesn’t even apply in the
large source like a chorus case is a link to a Bruce Bartlett article on ProSoundWeb.com which states: “The 3:1 rule cannot be applied to miking a choir with a few mics. Why? Most of the singers are somewhere between the mics, and those singers will be picked up with some phase interference. However, since each singer is in a different position relative to the mics, each singer is heard with a different coloration. The effect averages out over all the singers and so is not very audible.”
I often see it stated that the rule applies to mics that will be summed to a single channel, ruling out stereo.
Isn’t the whole rational for the rule to reduce comb filtering issues? Usually I see the rational stated as something like:
“This creates a level difference of at least 9 dB between microphones, which reduces the comb-filter peaks and dips to an inaudible 1 dB or less”. Spacing your AB pair that much will certainly do that, so much so that the phase information between the two will be nearly uncorrelated.
None of the first three pages of a Google search turned up links which apply it to stereo mic’ing.
Oh good, quitin' time. I’ll shut up now. Apologies for the overly long post.
