If I take a first response pregnancy test 3 weeks after sex, can I be sure its accurate?

More or less. If you do the test correctly, the results are likely to be accurate if the result is positive. If it is negative, you should repeat the test in 4-5 days. Use first morning urine and follow the instructions for the test carefully. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex. Get HPV vaccine.