Eb when in G is a flattened submediant substitution. It's also a bastard to fret properly.
Certainly, there's nothing wrong with thinking about chord substitutions, and the notion of taking equivalent minors for major chords is a good place to start (G - Eminor, C - A Minor etc)
Also, the notion that the three chord trick is based on I, IV, and V, is worth challenging: Western harmony is based on Tonic:Dominant relationships. V pulls to 1, therefore, what pulls to V? II! which is Equiv Minor to IV! Ta-daaaaa! instant calypso.
Whilst on the subject of harmonic substitution, Jazz types might want to replace chords with those a tritone apart, i.e. C - F#, G - C#, F - B etc
I suspect that this is getting a bit far from the thread title though. There's a nice G you can get by putting two fingers on the 3rd fret E and B strings, and not playing Low E and Low A.
Hope that helps.