Dang Yankee hit it on the nail head. if your playing an A major chord (triad) as long as there is a 1,3 and 5, or an A, C#, and E you are playing the chord or (triad). So if you miss the high E string you have an E (5th) on the D string. Also practice the A major chord using three fingers. I believe Guitar Tricks has a lesson or two on triads, this will help makes things a little clearer for you.
Don't give up: it's not a race, it's a journey.