Why wont my car start when it has a new battery only 3 months old

Suggestion for the future, put ALL information and questions in the same post. Sounds like a failing belt or alternator (or both), may be a cause of your dead battery. FIRST, have the battery charged properly ( NOT BY JUMPING) , then start the car and observe--. Charging SYSTEM can be tested most accurately when the battery has been fully charged. If you just jump-start it, the results most likely sill be inaccurate.