Hello Good Evening,
If you have done sex just 3 days before the due period date, than very rare possibility to get pregnant or can say that is nil, because ovulation occurred around middle of your period, for example in 28 days period, it is around 14th or 15th day.
But in different females, due to multiple factors, it can occurs 9th to 22nd day of the period, so you are looking pretty safe to conceive.
And if talk about your different route of sex, than you think how can it possible to contact between egg and sperm, because sperms are in anal canal
and egg is in the ovary or Fallopian tube
, so totally different places.
So be relax about your pregnancy
and just wait your period to come, to confirmed that you are not pregnant.
Dr. Naresh Jain