The 3:1 Rule

When it comes to studio recording there is one trick that every beginner recorder is often taught, which is known as the 3:1 rule. To break down this rule in general it is pretty simple. When recording the distance that is from the first to the second mic should be at least three times of the distance from the source to the location of the first mic in general.

One common misunderstanding of the 3:1 rule, however, is that it applies to stereo mic-ing of a single source. Untrue. In fact the most set-up for stereo mics is to have the mic ends practically on top of eachother, but pointing in different directions. So the 3:1 rule would totally negate that.
Where the 3:1 rule applies is when you plan to use multiple mics on the same channel. The different distances will reduce phase problems between the 2 mic signals. In stereo recording, you need to pan the two recordings from the 2 mics, so the phase differences are not really an issue at that point. Also, the 3:1 rule most often applies to mic-ing multiple sound sources (drum kit, 2 singers, etc.) rather than a single one.
Key Takeaways:

• I would certainly have used some outboard gear and other mics better adapted to the situation, but always with the same minimalist approach of reducing the number of mics to the strictly necessary.
• An urban engineer friend of mine once told me that it’s extremely rare for people to turn around when they’ve realized they’ve gone down the wrong road.
• Something similar happens when you have several tracks for a single instrument.. I mean you already have them, so why not use them, right? Well, because it isn’t always for the best.

“The 3:1 rule is pretty basic: the distance from the first to the second mic should be at least three times the distance from the source to the first mic.”

http://en.audiofanzine.com/recording-mixing/editorial/articles/the-3-1-rule.html