I know from experience it's hard to imagine how to take on 3 strings with 2 fingers, so this image shows an example: i.imgur.com/s79i9MV.jpg The index finger holds the 4th and 3rd strings, and the middle finger holds the 2nd string. Since I never could fit 3 fingers in one fret on 3 adjacent strings, this is how I always play an A chord.

@No'amNewman It's not: the third chord has a G in the bass, but a D in the melody. This sounds harmoniously, because D is one of the notes of the major chord on G (G B D).