Which IC can i use to connect three microphones to have a single output?

Use a 4 way selector switch - double pole or single pole - single will do.

Connect the output to the IC as required
Connect the Microphones as follows

Position 1 - no microphone connected to the IC
Position 2 - Microphone 1 connected to the IC
Position 3 - Microphone 2 connected to the IC
Position 4 - Microphone 3 connected to the IC

This is a mechanical arrangement - if you wish to do it electronically there needs to be some way of deciding which microphones are "on" - microphones are always on - it's just that sometimes they're just not connected.

Good Luck...
December 2015