R.F.
My daughter will be 5 in June. She will suck her thumb when she is tired. I know my dentist is not totally pleased with it, (but my pedi really isn't concerned) but I would rather let her grow out of it on her own than to give her a contraption or some liquid on her thumb to make her quit. I know she will quit before college :).
3 months old - that's just a comfort thing.
FYI - I was apparently 'addicted' to binkys when I was little - my mom told me I had at least 4 in my crib. Drop one, pick up another :) Binkys are easier than thumbs. Can't throw away a thumb!