I really would like to know because Soma and Erin are mean to each other and I want to see her respecting Soma.
Comparing to the synopsis of the anime episode on Wikipedia, Erina found out about Soma's dad on season 3 episode 12 (episode 49), "The One Who Aims for the Summit".
[...]. Soma then tells Azami that Joichiro is his father, right as Erina walks in on the conversation, shocking both of them. [...]