Not on BC-Last MC 10/18 w/mid-cycle spotting 11/3-11/6 (1st time EVER); now 12 days late, experiencing pregnancy symptoms, but 3 HPTs neg; am I preg?

Blood test for HCG. If you did the test correctly and it was negative at 12 days after a missed period, you not likely to be pregnant. You should get a blood test for HCG to settle the issue.
If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD.
Practice safe sex if you have sex.