Convert stereo to mono

i have a matrx 3x2M which is a song in 3 channels, the sound function doesn't reproduce more than 2 channels and to listen to it i need to export it as wac (lately even this method doesn't work) is it possible to convert 2 of this channels to a single one while sounding the same, and after that add the third one and play it as stereo??

[Rik]

Human perception is seldom linear, but you could try using the mean of two columns as a first approximate.