There are some situations where the ‘3 to 1’ distance rule isn’t practical or workable. Some that come instantly to mind are drum overheads, choir, string section, etc.
How can you deal with these situations where close micing with more than two mics is impractical for your needs (choir, string section) or unworkable (drum overheads) in a way that eliminates, or at least greatly reduces phase cancellations?
The answer is much easier to implement that it is to try to explain without visual aids, but since I’m artistically inept, and don’t happen to have two mics handy to take a picture, it will have to do for now.
The answer is called an “X-Y” pair. You use two identical mics of your choice, and place them so one aims toward the left at an angle of 45 degrees, and the other aims toward the right at the same angle. Place the mics so the capsules are as close to each other as physically possible without touching (to prevent vibrations from causing them to rattle against each other). You can vary the angle of the mics to change the amount of stereo spread (assuming you are mixing in stereo). Angles wider than 45 degrees will give you a larger stereo image, while narrower angles result in less stereo spread (even when you listen in headphones!).
The reason the X-Y pair works is that, while you have one mic aimed to the right and one to the left, the capsules (the part which actually picks up the sound) are so close together that sounds reach them at the same time. They are said to be “co-incident.” This results in very little phase cancellation problems, as well as a very natural sounding stereo image.
When using this for drum overheads, start with the pair almost directly over the drummer’s head. Move the pair left to right to get a balance on the different cymbals. Moving the pair from front to back generally changes the tone of the cymbals. Raising or lowering the pair can usually change the amount of toms and snare you get in the overheads in relation to the amount of cymbals.
As I said earlier, an X-Y pair is also a good choice for a large choir, or a string section. You can also try it if you need to mic a grand piano in a studio (probably not in a live setting since there will be too much leakage from other instruments).
If you need to have a “wide” sounding guitar track in a studio, us a close mic on the cabinet and pan it in the center. Use an X-Y pair some distance away (whatever gives you the sound you’re after), and pan those as far apart as you want.