A CERBER3 file is a virus file that is used by the Cerber ransomware virus. It contains a user's file that has been encrypted, holding it ransom until the user pays the creator of the virus.

The Cerber virus is packaged as a trojan horse in order to trick the user into installing it on their computer. It may be distributed through a fake website or a link in an email. Once the virus is opened and installed, it takes over the computer, encrypting files (documents, pictures, videos, etc.) and appending the "cerber3" extension. The files can only be opened once the user pays the ransom, usually bitcoins, to the creator. Varioous security programs can detect and remove the Cerber virus. However, these programs are most likely unable to recover the files.