My baby is 3 months now when she is full she can belch with a lot of milk that comes out. She does this every time either full or not. What do I do to prevent it?

Sometimes it is normal for this to happen. Children at times do not want to let go so they over feed especially when breastfeeding. Try to place him on the shoulder to burp (that is remove gas) otherwise it causes the milk to come out as they belch. If it too persistent sees your paediatrician just to confirm that there is no other under lying problem.