Interestingly, in Australian military jargon, the term is a nickname for the 3 RAR (3rd Battalion, Royal Australian Regiment), as noted in the Australian English military slang glossary . If your query is related to military history or a specific unit report ("write-up"), it may be referencing this battalion.
Searching the binary data of the file for readable text. cats 3.rar
If you are currently working on this file, here is the standard process used in similar challenges: Interestingly, in Australian military jargon, the term is
Often, these files are password-protected. Common steps include: Checking the challenge description for clues. If you are currently working on this file,
Knowing the source will help in providing the exact flag or solution steps. Appendix:Australian English military slang - Wiktionary
Running a tool like john (John the Ripper) or hashcat with a wordlist like rockyou.txt .