Number of factors. There are a number of factors to consider. First have the child's hearing tested. Other factors are how many other children you have, what is this child's order of birth, how many languages are spoken at home, and are the other developmental milestones (motor and personal social) age appropriate. Speak to your pediatrician about this. Ask school for evaluation.
Not normal. See the doctor to assess overall development including hearing.
No. By three your child should be talking in sentences with 75% of all words understood by strangers. If you are in the US you can take advantage of a program called "child find" in public school districts. This program looks for kids 3 and up that have any issue that might impair school performance at school entry. If not readily available, find a speech & language center for an evaluation.