Should you trust the negative if a pregnancy test is negative three weeks after any bleeding has occurred?

Home pregnancy tests can be positive as soon as the first day of your missed period. Confirmation of pregnancy can involve additional testing such as blood test for hCG or a transvaginal ultrasound, typically done at 5 weeks to see the fetus in the gestational sac. If you have tested after the first day of your period/missed period, then it is conclusive.

No, if the conception happens between the second week and third week (post last bleeding), urine pregnancy test should not be positive on week 3 because it is too early.