I think it's because we can scan it easily. Groups of 4 or more can confuse the eye: when looking at 5 things, you sometimes have to take a moment to realize i

I think it's because we can scan it easily. Groups of 4 or more can confuse the eye: when looking at 5 things, you sometimes have to take a moment to realize it's 5 not 4 (and, to a lesser extent, the same is true of 4). groups of 3 have first, middle, last - crucially _one_ middle digit, not 2+, which makes for quick comprehension.