a good attempt would be a basic knowledge in music theory - especially in basic chord structure. i know alot of bands play harmonyparts with paralells, but that does sound too one-sided to me.

if you wanna aproach it differently, then try and pick any note and see what other notes are sounding good along with it (major/minor thirds, 4ths, 5ths, octaves, etc) - with 3 guitars i strongly recommend the above mentioned aspects of music theory - and if you want examples in sound, then go and find midifiles of classical music - especially quartetts written in counterpoint (4 seasons for example) - a few soundexamples are in my host-section on this site - go and check it out.