It refers to a trick that can help to keep your recordings or performances sounding clean and as phase-free as possible when you're using multiple microphones at once to capture mono audio. 3:1 refers to the ratio you can use to figure out where to place your second microphone. When you use the ratio, your second microphone should be three times the distance from the sound you're recording than the first microphone is. The increased distance between the microphones helps create a volume difference in the audio captured by each microphone, as the signal captured by the microphone is lower. According to DPA Microphones, setting your microphones up using the 3:1 rule should create around 10 decibels of difference between each microphone.