Watch the Shapeshifter online guitar lesson by Brad Carlton from Chord Studies: CAGED Chords Vol. 2
This is a vi - ii - iii progression in the key of C. Our Am chord utilizes the Dm form at the 7th position. With the Dm you're going to use the Gm form, and with the Em you'll use the Am form. These particular forms are very useful because on the top string you've got some great chord tones in a great register for rhythm guitar.