The advice about a 3 months window period applies only to the HIV
antibody tests, not the duo test
(for both HIV
antibody and p24 antigen), which is definitive at 4 weeks. Below is a link to a thread that discusses window periods, and the reasons that official advice remains 3 months despite scientific evidence that earlier testing is conclusive.
The oral swab self test is pretty much fool proof. That result also is 100% reliable.