I play the open A with just my index finger on the B, G and D string on the 2nd fret. This makes it easy to wrap around your thumb for the F# bass note or your pinky, ring, or middle finger for the G# bass note. I use my ring finger.
If you don't already play the open A this way it'd be a good idea to try it out. With practice you should even be able to eventually allow the high E to ring open.
I use just the last digit of my finger to cover the three strings.