If I had unprotected sex three days ago, is it too soon to take a pregnancy test?
No, even the most sensitive pregnancy test won't give you an accurate answer until at least 6-8 days after conception, and conception may not occur the same day you had sex. Here is a link to a good article that explains how pregnancy tests work, which includes a list of the various tests and how sensitive they are: Home Pregnancy Tests. It is possible for a blood test at the doctor's office to detect it sooner, but you would still need to wait a little longer. I know it's a nervewracking thing to consider if it's unplanned, or that waiting to know if you are pregnant if you're trying. But it does take time for the appropriate "flags" to show.
Here are some other links you might find helpful:
A Quiz on Birth Control
5 Myths About Sex and Pregnancy