Is it possible to get pregnant if you are on the third day of period ?

Pregnancy has been documented from intercourse on any day of a woman's cycle including menses. It is less likely that mid cycle but does happen occasionally. Those with irregular menses are more likely to ovulate at unpredictable times making pregnancy possible in the cited exposure.

Pregnancy is known to have occurred from sex on any and all days of the cycle. It this happened less than 72 hours ago, you may wish to take Plan B. Use contraception all the time, every time. Get HPV vaccine. Wish you good health! Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines