Actually syphilis can take up to 6 weeks, and hep C is very rarely sexually transmitted. The hep that is sexually transmitted is hep b, and that can take up to 6 months.
AJ
An HIV antibody test before 3 months is not considered conclusive. A 9 week negative is highly encouraging though and come at a time when most people who are going to test positive will have.
For each STI you should test after it's respective window period. 9 weeks covers some (Chlamydia ...) but not others (Syphilis 6m, Hep.. though C is usually sexually transmitted .