Most medical professionals calculate your 'weeks' from the first day of your last period... since most women know this date and not as many will know conception or ovulation date. Just makes a more universal base point to start from.
Congrats on the pregnancy!
that is the difference between fetal age and gestational age. GA is the 5 weeks you are speaking of which is the age of the baby PLUS two weeks. Fetal age is just that. How old the baby is. In your case, 3 weeks.