There's a song played for a short amount of time in Boku no Hero Academia season 3 episode 45 at 9:50. I think this song hasn't been played before in the series.
What is that song?
As another user pointed out, it has actually been played at least twice before!
It is "Kyōi to no kōbō" (The Threat of Offence and Defense), the 29th track from Boku No Hero Academia Original Soundtrack. (YouTube link)