Am I suppose to get my period 3 days after I had unprotected sex and how will I know if I'm pregnet since this happened

Well you either won't get your period, and you'll get bigger around the stomach area. OR you'll take a pregnancy test and it will come up positive.
There's only so many ways to tell your pregnant,not to be harsh but most are pretty obvious.

I will assume you hope you are, since you were having unprotected sex, and yes there is a good chance you could be. But you may not, until you take a test it is not possible to know.
Good luck

Wait one week until after your period is due and take an at home pregnancy test.Use first mornings urine.