Mixpre-3 : 3 Preamp | 3 Channel | 5 Track Audio Recorder
Mixpre-6: 4 Preamp | 6 Channel | 8 Track Audio Recorder
MixPre-6 can send eight channels (1-6, L, R) to the computer and receive four channels from the computer via USB, while the MixPre-3 can send five in (1-3, L, R) and receive two out
It seems to me that they could enable five channel recording. The aux inputs don't the kashmir pre-amps. While it's artifical, that they way they designed the 3, presumably to not compete with the 6.