One thing that it is really important to understand--REALLY IMPORTANT!!!!!!!!--is that each chord position is a universe of its own--

G is the only fingering that offers you an open major chord to work with on the fretboard.

E offers that big fat low note that you can play against--and, because of the open b and open e next to each other,offers the possibility of harmonizing your melody

A is the most flexible because you can play a two octave scale without changing position, and you can play a D chord and D scale by shifting the position of one finger--(you can play a walking bass line and lead without changing the position of your hand!!!!!)

Each has it's own range of possibilities, and each has it's own distinct sound--