Watch the The 80's online guitar lesson by Angus Clark from Essentials: Rock Rhythm Unplugged
This type of triad exercise will prove useful throughout your life as a guitarist. It is the linchpin of most pop overdubs, and in this context, it's the heart of your rhythm arrangement. Putting the capo on the third fret allows you to experiment with triad voicing across the first section in order to make the part really come to life.
The B section is another example of how making the right choice with the cap saves you from exhausting your left hand. If we had to play Bb and F bar chords during this performance, it would be considerably more taxing than what we have here, which is simply back to playing cowboy chords.
As always, the right hand is the engine, and your source for subdivisions and dynamics, so keep it grooving while creating some variety using accents, muting, and selective strumming.