Is incubation period different from window period in hepatitis B? If my HBsAg is negative 3 months after exposure, am I 100% free of the infection?

No. want to check hep b envelope, core as well as surface antibodies in addition to hep b surf and envelope antigen. If all neg at 6 mo then no further testing needed. If hep b surf ag and hep b e ag and core, igm surf ab neg at 3 mo then very good sign If concern for hep b then probably also concern for hep c and hiv. Testing for these should also be done. Hope this was helpful. Best of luck.