[Crabby]

@Douglas S as @Loraine says do what is most comfortable for you.

However If you watch carefully you see he ways to use the ring finger on the picking hand only on the thickest level of strings. As he moved to the thinner strings he said use the middle finger. I think he simply misspoke the first time he said it.

I think he intends for us to use thumb index thumb middle on the picking hand all fo the time throughout the exercise.