Huge problems creating an aggregate audio device on separate channels from mono input mics

I'm currently trying to record from 3 separate Sennheiser PC7 headsets which are plugged into my MacBook Pro. Because the mics on these are mono, the Audio Midi Setup is detecting them as only inputting Front Left and when you create an aggregate device with 2 mics, GarageBand records one mic input through the left channel, and the other mic input through the right channel, on the same track.

I need to be able to split the mics up so they're playing on their own individual tracks. Does anyone have any suggestions? GarageBand has well & truly defeated me and taken my soul here.