Could you be pregnant but still have a period. I had unprotected sex but about3 week later I had my period?

It's possible, but it's not a real period, just spotting.
ANYTHING is possible with pregnant vs not pregnant.
Some women have 1, some have more, and don't even realize they are pregnant.
There's no point in finding out odds. Let's say it's only 3% - you are 100% pregnant if you are in that 3%.