Here's a worthwhile exercise in spicing up an arrangement. It's elementary theory, but it's practical theory - you don't really have to understand what you're doing as long as you have a good ear and good memory.

Take the three tones of the major chords that you are using. They are the tones that match Do, Mi, and So (of do,re,mi,fa,so,lw,ti,do).   

Do is the root, Mi is the 3rd, and So is the 5th.

Once you find where the three tones fall in your chord, start to experiment with what the chords sound like if you move the third up or down a fret oe two.

Now try moving the fifth up or down a fret or two.

Just experimenting this way will lead you to understanding ways that you can spice up three chord songs. Some of the "new" chords that you find this way will suggest new songs or where they might fit in a song you already play.

You will also see how the subtle one note changes to a chord will flavor it.

Besides, it's fun.