Oo, December 3 is counted as one month na! So kung November 3 ang last period mo, then December 3 you're already 1 month pregnant, that's how they count it in pregnancy, based sa first day ng last period mo. Best to do a pregnancy test now or see your OB to confirm and get proper dating, kasi maraming moms kami dito na nconfuse din at the start pero mas clear na once you talk to the doctor!