Watch the Day 10 online guitar lesson by Ravi from Learn Guitar in 21 Days
Let's stick with three chord songs in our I-IV-V progressions, but we can now do the same songs in the key of A (A-D-E) and in the key of D (D-G-A). So, if a three chord song didn't sound quite right before in the key of A or wasn't great for your singing range, now try it in the key of D (use D instead of A, use G instead of D, and use A instead of E).
We also have a couple rhythms to use, so try some with a folk/pop and others with a rock/blues. Experiment, you have complete artistic license!