Can you get pregnant on your 3rd day of your period?

Yes: Anyone having sex without condoms for birth control is at risk of getting pregnant. During your menstrual cycle is a less likely time but certainly still possible if ovulation occurred at an unexpected time. If you don't want to be pregnant use condoms for birth control. If sex was within 72 hours use plan b. Consider seeing an Obgyn to discuss long term contraceptive options

Yes: The likelihood is very low as, presumably, ovulation is in the past. It is important to keep in mind that while the signs of hormonal fluctuations are generally observed in the form of a cycle, absence of one, bloating, cramping, etc..., these are not absolute indicators. Any act of unprotected vaginal intercourse is a risk factor for pregnancy.