The 3:1 rule helps prevent phasing issues.
What’s the rule? When using two mics on a single source, the distance of the second mic from the source should be three times the distance of the first mic from the source.
How can you apply this rule as a recording drummer? If, for example, you place an overhead mic 3 feet from your snare, and you want to add another room mic, the second mic should be at 9 feet from the snare, in order to avoid phase issues.