The third from the left is a member from BTS (Bangtan Sonyeondan) Jin is called like this because on a picture he was the third from the left. A lot of people saw that picture because it was from the BBMAs (Billboard Music Awards) But they didn't know his name so they called him the third from the left.
"The boys are good looking. But who is the third from the left?"
"The third from the left looks handsome"
"I like that guy! The third from the left"
That are real comments below the photo.