Practicing Multiple Chord Changes
For a song that has more than two chords you still want to work on each pair of chords to smooth out your moves before moving between all three. I recommend approaching it in such a way that you can move between any of the chords involved smoothly before all is said and done, rather than only focus on the particular moves of one song. If the song you're working on only goes from D to G and G to A, don't only work on those pairs. Work on being able to move between D and A as well. By doing this you are, in essence, getting a head start on any future song using those three chords.
So, below are exercises to work on all the combinations of D, G and A open chords as a chord progression. Once you can do each of these pairings smoothly you should have no problem playing any and all sequences you may encounter involving these three chords.