A crowd may form due to unintentional situations, such as accidents. For example, at a car crash, people crowd around the scene to see what happened. A mob is something like a gang that work together to achieve a common goal which is negative. For example, the mob robbed the rich lady of her purse which was stashed with money. A gathering usually form intentionally, such as in a Science classroom, the teacher wants to demonstrate an experiment and asks everyone to gather around the apparatus set up so that students get a better view. That is the difference between those 3 words. However, they all have something in common, which is many people coming together becuase of something.