Averaging electret microphones' inputs
The output of an electret microphone element is a small (millivolts) AC voltage riding on a large (volts) DC voltage.  Because no two elements have the exact same DC rest voltage value, a better way is to capacitor-couple each of the three signals to a 2-resistor voltage divider between Vcc and GND.  This is then connected to the ADC input.  In this way the three signals are summed to a single signal that is the algebraic average, and that signal is biased to the middle of the ADC input voltage range.

Are the microphone elements 2-lead or 3-lead?