Scotsbard, my fingers are a bit too thick to do an A with three of them, so I use my ring finger on the B string and my middle finger, bent backwards, on the G and D strings (ooMMRo). Since the B string is fretted, the normal tendency of the middle finger to foul it when used this way is moot. It also makes for a super-quick change to A7 (ooMoRo), just let that middle finger "stand up".