WHILE YOU CAN PLAY MOST SONGS with just three chords, you get a lot more flavour if you add the relative minors of each of these chords. These are chords three notes below each of the main three. The relative minor of C is Am (the notes are A, C and E); the relative minor of G is Em (E, G and B); and of F, Dm (D, F and A).
One of the best songs to try this out on is Postman Pat, reproduced here in the key of C.
Despite sounding like a simple tune, it sneaks in a couple of surprises. The first is the half-length bar in bar 4; the second is the sudden jump to the chord of E major in the fourth line, which adds a piquant flavour.
At the end of the third line the tune rises in semitones; if you add a third beneath, as shown, you get a harmonious bridge.