Watch the Day 5 online guitar lesson by Ravi from Learn Guitar in 21 Days
Yes, you know three chords now, but let's focus on the new ones, D and A (both are "Major" chords). Take some of those same songs you were playing with A and E, but instead of A, play D, and instead of E, play A.
Take your time, this might be a bit confusing at first because you are breaking some new ground. However, because you are already familiar with some of these two-chord tunes, switching to D and A will come quite quickly.