The 3 to 1 rule states that the far mic needs to be at least three times the distance away from the source as the close mic is. 

The most common use for it is when there are multiple instruments playing in the same room, each with their own mic.  Like a trumpet player and a sax player in a room with thier own mic.  The sax player needs to be at least three times farther away from the trumpet mic than the trumpet player is, otherwise you will end up with phase cancelation due to bleed. 

The same thing is true for drums.  The snare needs to be at least 3 times closer to the snare mic than the tom mic. 

The other use is using a close and distant mic on the same source.  This is the one that seems to give people the biggest trouble.  Because it only works as long as the two mics are at equal gain (not equal volume).

The 3 to 1 rule is about isolation, not phase.  The 3 to 1 rule can make it easier to avoid phase problems, but only because it isolates one mic from the other.  It's the isolation (the lack of bleed) that keeps the phase problems in check, not because it's some magic point in space where all the frequencies become time-aligned. (that can't happen)

As long as the two mics are at equal gain (not equal volume) the signal coming out of the distant mic will be 9db lower than the close mic.  That difference in volume makes the phase cancellation (which is still there, by the way) so quiet that it isn't a problem.

However, if you match the volume of the close and distant mics, you will have all the phase problems you would expect.

There are limits, of course.  It won't work if you have a zither in the same room with a raging Mesa Boogie stack.  The sources have to be relatively equal in volume.

One last thing.  The 3 to 1 rule does not apply in any way, shape or form to any stereo micing techniques.  The phase shift is one of the things that makes stereo work.  Keep in mind that phase is a time shift and is not to be confused with polarity.



Have a nice day