Got negative pregnant test 3 weeks after lmp, a week later tested positive. would this be accurate or late ovulation? time of conception unknown.

See below: Sometimes urine pregnancy test can be falsely negative especially if your urine is very dilute. If in doubt, a blood test will confirm it.

Could be: Many pregnancy detect pregnancy after 2 weeks of conception.

You are pregnant: A test will turn positive about 2 weeks after ovulation / conception. This is exactly what occurred. Conception timing will be confirmed after first ultrasound and after confirmation of gestational age.

Pregnancy tests: A pregnancy test may be positive before the first missed period, but not always. It may be the test's sensitivity or a delay in ovulation.

Accurate: It seems to be the correct time for pregnancy test to be positive, but if you want to be 100% certain repeat the test.