If you try it out with your own hand, you will find it awkward to indicate a number three otherwise. For example, if you try to force the pinky down and show three with index, middle and ring finger, the ring finger won't extend, because the pinky is down. So, it is easier to do it like Jim is doing. It would look pretty weird to use the thumb in this equation also.
Do an image search. By far the most common 3 fingers to hold up are the index, middle, and ring. But even that is not the point. He started with the most common, and then switched to the pinkie, ring, and middle. This is very uncommon, and that is exactly why it was mentioned. It's easy to hold up any 3 fingers, but hard to start with 2 up, like he did, and then switch.
If you try it out with your own hand, you will find it awkward to indicate a number three otherwise. For example, if you try to force the pinky down and show three with index, middle and ring finger, the ring finger won't extend, because the pinky is down. So, it is easier to do it like Jim is doing. It would look pretty weird to use the thumb in this equation also.
Do an image search. By far the most common 3 fingers to hold up are the index, middle, and ring. But even that is not the point. He started with the most common, and then switched to the pinkie, ring, and middle. This is very uncommon, and that is exactly why it was mentioned. It's easy to hold up any 3 fingers, but hard to start with 2 up, like he did, and then switch.