Self-taught originally, with a formal teacher later on…
A: I always do A with 3 fingers, but vary which fingers depending on what the preceding or succeeding chord shape is. If there’s a D, I usually play A with Index/Middle/Ring. If no D, I usually play with Middle/Ring/Pinky. Sometimes I’ll barre with one finger if I’m sliding down from a B, but that’s rare.
G: I usually do 3-finger, but also do G5 frequently (nice subtle accent in a lot of strummy songs like Pink Floyd)
"The four-finger G (320033) — which I’ve most often seen called a G5 — works better for a lot of rock, and especially power pop. If you want the third in the lower register, you can do this, or you can mute the A string if you want a true G5. "