Sucking the thumb or anything goes near the mouth is normal and is due to the neonatal rooting answer suckling reflex which is nature's way for survival. At 3 months the teething process also starts so the baby gets comforted by biting on anything that's in his mouth with his teeth that are still enclosed within his gums. Sucking the hand will not reduce the feeding though. When the child is hungry after 2 to 3 hours of the previous feed , you can offer him a feed and he will take it. To reduce sucking the hands , you need to distract him or keep his hands out of reach.