Hi Nihar first of all thumb sucking is harmless, few kids does this for comfort and they have full control over it, So better than pacifiers. When they suck they do have action along with that like few will catch tip of the cloth, few will fiddle their nose, hair , hold on to something etc. It's just a habit, As he is 3 yrs old he can very well understand so say it in a good way, Never shame him infront of everyone. Tell the family to ignore, the more they tell the more he is going to do. There is a timeline to wean everything, I myself sucked my 2 fingers till 4 yrs but as I realized other kids at school did not do i stopped. Applying something or tying his hands will give bad effect on his mental health so never do that. Do not worry its just a habit and with patience make him understand. It's definitely not a bad thing.