Merge voices in Garageband?

Hello,

I currently have three different tracks in garageband. Im making a computer voice (which I want to use on Skype). I have managed to create the three tracks, all with their own pitch, but I then realized that only one track can be used as a "voice changer".

I have one "helium" voice, a normal voice, and a very deep voice on the three tracks.

I have tried putting all of them on a single track, but found that the pitches cancelled each other out, instead of having a mix of all three.

Is there a way to select all three of them as my voice changer, or do i have to use a single track. If so, is there a way to put all three on a single one, without having them cancel each other out?