Anytime you have unprotected sex you are at risk of pregnancy. Conceiveing three days after your period dose increase your chance as you are probably ovulating again.
its a very good chance she could get preg sperm can live in her for about 5days anytime your haven unprotected sex you are taken a chance of getting preg and std so please bye some condoms are go to a health department thay give them out for free.she can test to see if shes preg after her miss cycle.good luck to u both.