what food should be given and in how much amount to my labrador of 3 months

Question
what food and in how much quantity should be given to my black lab of 3 months

Answer
Most of the common brands of adult chow would be fine now.  Introduce the new food slowly over about a week to avoid digestive upsets.  Start with 1 1/2 cups each of 2 times a day, a little less if the puppy is over weight.  Nobody can say exactly how much.  Even at 3 months, your dog definitely should be narrower at the waist than the hips and chest. You should be able to easily feel the ribs, but not see them.  Each dog is different.  Standard recommendations are a good place to start, but each dog must have its food and exercise adjusted to its individual needs. Here is a link to a good illustrated guide, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx

Puppy chow is formulated for maximum growth.  Large breed puppies are better off on adult chow, growing slower and having more time to develop sturdy joints.