What is the main point of the three parables of luke15?

The Pharisees and scribes resented the fact that Jesus fraternized with people who were obviously sinners. The Pharisees and scribes did not show grace to these social outcasts, and they resented Jesus' doing so. They complained, "This Man receives sinners and eats with them." This was true, of course.

It was in answer to their complanit that the Jesus recounted the parables of the lost sheep, the lost coin, and the lost son. These stories were aimed directly at the scribes and Pharisees, who were never broken before God to admit their lost condition. They were as lost as the publicans and sinners, but they steadfastly refused to admit it. The point of the three stories is that God receives real joy and satisfaction when He sees sinners repenting, whereas He obtains no gratification from self-righteous hypocrites who are too proud to admit their wretched sinfulness.