A song in Boku no Hero Academia season 3 episode 45 at 9:50
The Threat of Offense and Defense (脅威との攻防 Kyōi to no kōbō)

As another user pointed out, it has actually been played at least twice before! 


season 1 episode 10 at around 10:50
season 2 episode 3 when showing everyone on the minefield