The ASIFA blog asks one of the great ineffable questions, “Who invented the three-fingered hand?” An essay by animation legend Grim Natwick explains:
Someone way back in the dark ages of animation got tired of drawing hands with four fingers and simply left one off, and cartoon hands have been much easier to animate ever since. It was a stroke of genius. The four fingered hand disappeared from animation until “Snow White” (1937). Somehow a pretty girl didn’t look right with only three fingers. But the Seven Dwarfs still had three fingered hands.
It wasn’t just the whim of a lazy artist.
I forget the book in which I first read it, but a quick Google search provides several versions of an old story in which Osamu Tezuka asked the same question of the Disney animation department. Why does Mickey have four fingers? Turns out the answer was purely technical:
“Because if you animate him with 5 fingers, it looks like he has 6 fingers when he’s moving.”
Here’s one version of the story: http://comipress.com/article/2006/03/03/2
The answer can be found by reading Three Fingers By Rich Koslowski. (Google Books has a preview. I recommend the book highly.)