Not much is known on how it classified either. However, logically, it is less serious than a Biohazard Level 4 Contamination.
To stop a level 3 contamination, the section is closed off after forty seconds of the alarm being triggered. Poisonous gas fills the room in order to kill any living organism whether infected or not.
The poisonous gas is then automatically vented out of the room and the section can be re-entered after the all-clear is given.
In the upstairs laboratory, several Albinoids were released. After a Bandersnatch kills a researcher in front of Claire Redfield, they hide away from view. Later, Claire visited the lab, she was attacked by them but managed to escape.
After the researcher was killed, the computer-initiated alarm went off. This caused the emergency shutter on the ground floor to shut. Luckily Claire managed to get under the door before it locked her in.