If I had a baby on March 3, 1991, what day did I get pregnant on?

That's pretty impossibly to tell, unless you know without doubt the exact days you were ovulating etc. Why does it matter now? If you are concerned over who the father might be, you'll have to take a paternity test to know that

As a labor and delivery intern, I can tell you that it is impossible to tell what day you conceived on. As Buggage stated, if you are worried about who the father is, you will have to take a paternity test now.