The night time bottle is the worst, but it usually affects the front teeth the most. Three is a good age to start brushing their teeth and teaching them how to do it. I would take your child to the dentist for an evaluation and make sure he/she isn't lacking in any vitamins or minerals. You want to take care of this as it can affect their permanent teeth. I hope this helps and best wishes.
It is common problem in kids generally with infants one, to protect them from tooth decay you can keep wet cloth with you and after each feed just wipe the mouth with it, this is the best prevention from tooth decay.