Thank you so much for checking out the "Right Hand Technique" lesson!
Yes, I use mainly the index finger & thumb pick for the single note lines/licks but I use the middle finger for rolls, open string runs, some triplet runs etc.
If you're asking about those triplet runs where I play same string twice and then next string, and next string again - the order is this:
The 1st note is played using index finger,
2nd note (same string) is played with the thumb pick
then we play index finger on the next string,
then middle finger on the next string,
then ring finger on the next string,
and finally ending with the thumb pick on the same string.
This might be a bit confusing explained this way, so here's a TAB for a lick that I use at 0:57 in this video: "Cowboy Solo"
Make sure to look at the right hand fingerings, and slow down the video. (By the way, this is part of the lesson "Open String Licks Vol.2" if you'd like to check it out. I slowly walk through all the techniques and ideas in that video)
p=thumb, i=index finger, m=middle finger, a=ring finger...
Regarding the descending triplet runs, I actually use the index finger to "sweep" across two strings. Se this lick and link:
"Reel Lick #22"
Let me know if this answered your question! I'm always here to help!