Me and my bf had sex a day after my period and he finished inside 3 times . i read i can't get preg but everyone tells meh otherwise . is it possible ?

Depends on cycle: It is possible to conceive by having sex up to 6 days before ovulation. So if you are having periods every 21 days then you are ovulating about 7 days after your period starts and that means you would have had sex within 6 days of ovulating and could conceive. If your cycle length is closer to the 28 day average for most women then the sex you had would have been more than 6 days before ovulation.