There is not enough room in the chassis to add a third channel as the 440 has four output transistors per channel (the HC version has six), and they use all of the available heatsink space.
We do offer the Insight 240/3 amplifier. The 240, being lower power, uses two output transistors per channel; therefore, we have enough heatsink space to make a three channel version. We also offer the double device option on the Insight 240/3 for those that want to drive a bit more difficult load (speaker).
Thanks,
Larry