Why do I have three eyelids? (Photo)

I'm really curious as to why I have three eyelids, among all my friends, they only have either double or non at all. I only realised it in 2017 when I didn't think much of it. I only started to wonder why when my friend pointed it out again . I use to have a double eyelid on my left eye, but still three on my right, and suddenly a few weeks later I had three. I hope you can explain it in a way I can understand. Thanks!