Honestly, it really depends on kung kailan exactly ang ovulation niya, 3 days after period, maaaring pregnant pero usually mas mataas ang chance around mid-cycle (around day 14 if 28-day cycle). For detection, most home tests ay accurate around 12-14 days after intercourse, pero para sa pinaka-reliable results, dapat 2-3 weeks or after na-miss na niya period talaga. I'd really suggest talking to her OB about timing kasi bawat babae ang cycle, and they can give better guidance, a few of us mums ask our doctors din pag may doubts about these things, alam mo na yan haha!