The “3:1 rule” is a general guideline typically used by recording engineers. It creates a ratio of distances between any two microphones based on the distance between the microphones and the sound source they are recording.
The 3:1 rule is used to reduce comb filtering where frequencies from multiple audio devices arrive at different times, causing some to be cancelled out while others are left alone.
For every 1 unit of distance between Microphone A (B, C, D, etc.) and Sound Source X (Y, Z, etc.), there should be 3 units of distance between Microphone A and any other microphone(s) (B, C, D, etc.).
This means that any given mike should be placed 3x as far away from the next closest mike as the closest mike is to the nearest sound source. In order to minimize the comb filtering effects, you will need to follow this rule for all combinations of mikes and sound sources.
The Principle Behind the 3:1 Rule
The 3:1 rule is a guide for positioning multiple microphones so that comb filtering will not exceed 1db. The principle behind the rule is based on the fact that the received signal strength falls off the farther the mike is from the sound source. The farther sound will fall off enough over the 3x distance to keep the phase cancellations from causing a comb filtering effect over 1 db.
If this rule is not followed, comb filtering can put very obvious notches into the received signal.
If mikes are placed equally far from a sound source, comb filtering will occur unless they are separated from each other by 3x their distance from the sound source.
This is why X-Y stereo miking involves putting the two capsules directly over/under each other. If they were placed side by side, then comb filtering would noticeably affect the sound until they were placed at the 3:1 ratio.