That’s perfectly normal at 3 months old! :) he’s discovering that he has hands. And he’s starting to explore with his mouth, which also helps him prepare for eating in the future. Plus, his gums may be starting to get uncomfortable in preparation for teeth, and sucking on the hand can be comforting. I would just let him be for now.
it’s perfectly normal as he’s exploring and learning to feed himself! just clean his hands whenever possible.
Cherlyn