Pregnancy is usually not detectable at 3 days.
Currently, pregnancy is established by a pregnancy test, i.e. blood or urine HCG (human chorionic gonadotropin), and by ultrasound to detect the presence or absence of a gestational sac.
HCG is the most common of these tests, and early pregnancy tests utilize this principle. Fertilized eggs can only be detected after they have been deposited in the lining of the uterus, so it is not possible to detect a pregnancy at 3 days of gestation.
However, pregnancy tests alone cannot be relied upon to establish a diagnosis of pregnancy. Positive blood and urine HCG tests can also occur in ectopic pregnancies and trophoblastic diseases. Therefore, an ultrasound should be performed to determine if the pregnancy is pregnant and if it is intrauterine.