Well, why not make it even harder? Why not create a song using only two channels? Could be an interesting exercise since 2 channel music is very useful in games since a whole channel then can be reserved for sound effects.
Not entirely true of course as the noise frequency is shared among the three channels sound effects can have influence on the sound of the other channels or vice versa no to mention in case hardware envelopes are used in the sound effects. Other than that it is impressive to see what you can get out of just 2 channels!
You know it is also possible to have 4 tones playing at once by playing two different notes on one channel using different frequencies for the tone and hardware envelope... I think this is supported in Arkos tracker II. Most of the time the result is not so good, but perhaps you can find some new challenge in there 
You know it is also possible to have 4 tones playing at once by playing two different notes on one channel using different frequencies for the tone and hardware envelope... I think this is supported in Arkos tracker II. Most of the time the result is not so good, but perhaps you can find some new challenge in there
Yes I explored that to create the bassline in my song Rezzy. Damn near impossible for higher frequencies though.
https://youtu.be/ol63MdcFhtc
I see (or hear 
). Other than some interesting high bleeps you probably found the best practical use there. I once made a 1 channel test song (Usas stage 1 cover) for my own tracker which sounds pretty bad.