should I buy plan B if I started BC for the first time and it's been 3 days?

I finally started birth control pills this past Sunday and today is Tuesday. So its been 3 nights I've already take them. I read you should wait a week before having sex without a condom and I even heard you should wait a month. Well it's been 3 days since I took them for the first time ever and my partner and I didn't use a condom oops. Should I get some plan B? I hate getting it bc it's expensive. Since it's already in my system, do I really really need plan B? I had bad side effects from it last time. If theres a high chance I'll be pregnant I will get it, thanks!