When should I take a HPT?

Implantation bleeding wouldn't occur this soon. It usually takes 7-14 DPO and since you said you only had unprotected three days ago it'd be too soon. Unless you had it a few times earlier in the week.... you really should contact a reproductive endo since they'll be able to help regulate or figure out why your period is so sporadic. If you want to test I'd suggest waiting 14 days after the day you had . Best wishes.