The instrumental music during Midoriya's monologue on My Hero Academia season 3's prologue
This one is about the first 30 seconds of "Combat Liscense!".

It's the 5th song of disk 2 for the third OST (My Hero Academia 2018 Original Soundtrack) 

Link to Amazon MP3 JP for preview, proof and a possible where to buy.

Note: track is about half way down under the Kanji 戦闘許可!