Only being 3 days late doesnt necessary mean you are pregnant. Its a little to early to do a home pregnancy test to get a good reading, so I would wait about a week then check it. Although home pregnancy tests can give a false-positive reading at times. If you had intercoarse around 7-10 days after your last period it will then be a possibility that you are pregnant, this is when you are ovulating. If it persists just call your GYN and see if there could be any other problems causing this. While your there have a blood test done to see if you are pregnat.