My 3 years child is with bronchopneumonia. What is the cause of this?

Many. There are many causes of bronchopneumonia in children. Of these, viruses can commonly result in this condition and if your child is otherwise healthy, it should typically resolve without long term problems. If your child has a prolonged cough for weeks or a fever, it is best to follow up soon. .
Cannot tell. It may be viral or bacterial, or potentially other types of germs. The best way in which to find out is to talk to the doctors caring for them and get the results of cultures if they have been taken. Hope they are better soon. Good luck.