It's the song that plays around 3:50 of s3 e9. I know it's been played on the show before. Another episode it was in was season 3 episode 1 when Todoroki was reminiscing at the pool. I believe I heard it before that as well. It is pretty soft and has a woman in the background.
Referencing against Episode 1 of Season 3 around 15:55, I believe it's included in the soundtrack for Boku no Hero Academia 2, Disc 2, Track 5: Kimi wo Koete (君を超えて).