My baby is 3 months old and he is sucking on his hand constantly, is it a sign for teething?

3. Although 3 months is a little early for teething, your child may be dentally advanced and may be teething already.
Yes, Children these days are teething early- drooling and hands in mouth not uncommon- some children have had teeth come in as early as 3 months and as late as 15 months. Don't forget to take you child in for their 1st check-up by the age of 2- caries are 100% preventable- it is a bacterial infection that need the teeth to adhere to.
Maybe. Babies will put lots of things in their mouths, even if not teething. This is normal. The first teeth to pop through are usually the bottom two teeth in the middle. There are special teething rings at most baby stores or you can go with the old standby of a frozen wash cloth to help your baby through teething.
Maybe. Although early, it is possible, also it may be a soothing mechanism for the baby to calm . Particularly if this does calm the baby down.