Why Was Lavender Brown Played by 3 Different Actresses?

One of the strangest things about Lavender Brown’s character is that she was portrayed by not one, not two, but three different actresses over the course of the films — changing between Chamber of Secrets, Prisoner of Azkaban, and finally Half-Blood Prince. But what’s the reason behind all of this? Why was she played by 3 different people? See Privacy Policy at https://art19.com/privacy and California Privacy Notice at https://art19.com/privacy#do-not-sell-my-info.