Can u get pregnant three weeks before your period?

Unlikely: Conception takes place within 24 hours of ovulation. Ovulation takes place 14 days before the menstrual period starts. You would not ovulate 21 days before your period - so you could not conceive then. You could have sex, then 6 days later ovulate and have about a 3 in 1000 chance of conceiving - that would result in conception occurring 14 days before the period due to sex 20 days before it.