The first rule of thumb is to use Iminor IVminor and Vmajor so in Am your three main chords are Am, Dm and E MAJOR. This rule is not as useful or as general as the three-chords-in-major rule cause there are too many MODAL songs which only use two chords-- the tonic (minor) and the seventh-- the note below it-- thus you can play Shady Grove with an A minor and G major and nothing more hard to tell a minor song from a modal song but try those two patterns and see how they fit! and have fun! PETE