Often, we must squeeze our fingers into some rather unaccommodating positions on the guitar. We know we are supposed to strive to get our fingers "right behind the fret" as we play, but, like so many things in life, we have to bend the rules (and the fingers!) sometimes (it ain't a perfect world!).
I have always found the A major chord to be a naturally antagonising chord shape. There is no way you are going to get all 3 fingers equally close to the fret. But, we do want to get them as close as we can, and to do that, we need to know that we must "overlap" the fingertips as they go down.
We don't want to allow them to line up side by side, as they tend to do, we must make them "overlap", especially 3 and 4. By overlapping the fingers, we get the fingertips as close as possible to the fret in awkward situations like this A chord.
Another common, and important playing situation where we need this finger overlap is in doing the basic 5th and 6th string root bar chords. In fact, we want to be able to make this overlap happen even above the strings, as you approach the chord.
Here is the common F bar chord done with finger overlap on 3 and 4.
Teaching a student to use "finger overlap" in bar chords....
This student's 3rd and 4th finger do not have the development yet to do the necessary overlap.
I am grabbing the students fingers and forcing them together, overlapping them. I am also bringing them as close as they can get to the fret.
This can be helpful, but what is really needed here is consistent practice of the Left Hand Foundation Exercises found in "The Principles Of Correct Practice For Guitar".
Here is an exercise for building the muscles in the hand that bring the 4th finger into overlap position.
Simply touch the pinky to the thumb. This strengthens the "Opponens Digiti Minimi" muscle, located in the palm. This muscle brings the pinky into overlap position.
Do this with the 3rd finger also, hold your hand out, fingers spread, and bring the 3rd and 4th fingers together, touching.
One last word, and again, this falls in the "it ain't a perfect world" category: people vary in the shape and size of their fingertips. It is easier to do the finger overlap if you have more tapered fingertips. Some people have more "bulbous" shaped fingertips, and it will make it a bit harder to overlap, but you can still get a little more finger in a little less space by doing the overlap technique.