Normally, women ovulate between 7-10 days after there period and if you had sex then you will have a chance of being pregnant. You need to do a pregnancy test at least a week after your period was suppose to start before doing a pregnancy test. This way you will get a good reading. Of course you can do a blood test 3 days after when your period was suppose to start.
It takes 6-12 days after the sperm meets the eg for the egg to attach to the uterus. The earliest a pregnancy test can determine pregnancy is 5 days before your missed period.