Watch the To E or not to E online guitar lesson by Brad Carlton from Chord Studies: CAGED Chords Vol. 3
This progression is a ii - IV - V in the key of G. Our first chord, Am, comes from the Cm form of the EDCAG system, and can be a bit of a stretch. Your fourth finger will be a pivot as you move to C5. This comes out of the D form of the CAGED system. Moving to the D chord, your third finger will act as a guide finger. This D shape is coming right out of your E form. Each of these chords have four voicings, and the open E string can work well over any of them.