There are a few rules-of-thumb in recording that are useful and time-saving. The three-two-one ratio rule is one of those that will assist you when you are setting up a recording session for multiple musicians, and it’s just as useful if you are miking a live concert.
One of the essential books for understanding microphones is Microphones: Design and Application, a 1974 book written by Lou Burroughs, one of the founders of Electro-Voice. This book is an excellent course in the basics of microphone usage, both in sound reinforcement recording.
One of the principles developed by Burroughs was the three-to-one ratio rule.
This rule is an empirical rule determined by listening tests for microphone placement and sets a minimum distance between microphones to avoid degradation in the sound from a given source. So, it is the minimum distance between microphones to keep noticeable bleed-through and phase problems under control.
The rule is: The distance between any two microphones must be at least three times as great as the distance between the microphone and the source of sound which it is to pick up.
So, for example, if you are miking a violinist and a cellist and the microphones are 2 feet away from their instruments, the microphones should be separated by a minimum of about 6 feet. I also believe that the rule implies that the sources will be of similar volume levels. Recording a screaming guitar amp in the same room with a quiet acoustic guitar is probably not going to work.
Remember, this is a guideline, not a precise law. But, keeping this in mind should make setup for sessions easier and help you avoid bleed-through and phasing problems between microphones.