Is there any thumb rule that is applied when playing 3 string arpeggios with regard to fingering pattern?
The way you worded made me think you were picking it with just your fingers. I thought you were asking if there was a rule for your actual THUMB that you were using. Anyways, I think its what feels most comfortable for you in the end, but using your first and third fingers on the first and second notes would make more sense so you don't have to stretch at all if you used your first and second fingers. That would leave you with a hammer on/pull off on the high E string with your first and fourth (pinkie) finger.