If a basketball player runs 15 m in a time of 3 s, what was the runners average velocity?

There's not enough information in the question to calculate the player's velocity. But we can find his average speed.

Speed = (distance covered) / (time to cover the distance)

Speed = (15 meters) / (3 sec)

Speed = (15/3) (m/s)

Speed = 5 m/s

There's no information telling us what direction he ran, so it's not possible to say anything about his velocity. ('Velocity' is not just a big word for 'speed'. It's a different thing, and to describe it, you need to know the speed AND the direction of the motion.)