Pardon me if I bring up the "over-literal" issue again, but consider the claimed contradiction outlined in this page. Some people might argue that the Hebrews verse is being taken too literally, and that what the author of Hebrews meant is not so much that Melchisedec really had no ancestry, but that his ancestry was not given in the text, which is a bit of an oddity. While that is a possibility, and probably the simplest explanation of this supposed contradiction, I think something more obvious is being overlooked.
Often in the SAB, a verse is taken from one place and another from a completely different book to point out a contradiction when the same point can be illustrated by choosing a verse in the exact same chapter. (Probably the best example of this sort of thing is Proverbs 26:4-5.*) I think often, when you notice that the contradiction exists right there in the immediate context, you should logically come to the conclusion that something else was going on than what you think. Maybe you think the writer of a particular passage in the Bible was stupid, but so stupid that they didn't notice something so flagrantly wrong?
Forget the book of Hebrews, or even chapter 14 of Genesis where Melchisedec actually appears. Stop right here in chapter 3 and think about what this verse is saying. Is Eve really the mother (i.e., female ancestor) of every human being? At the time Adam gives her this name and makes the claim, how many humans are alive on earth? Two. How many of them are descended from Eve? Whoops, zero!
Adam makes this claim knowing full well that there are already two exceptions to it. Why couldn't there be others? Genesis 3:20 is not a claim of absolute truth, but a general rule, and furthermore, one made by a man, not by God. Yes, there is a contradiction here, but it's not the Bible cointradicting the Bible, it's a person in the Bible contradicting a statement made elsewhere in the Bible. When Jacob says, "I am Esau," that's not a contradiction, it's a lie. When Eli says that Hannah is drunk, it's not a contradiction, it's a mistake. I'm not sure how you'd term Adam's claim here, but as far as I can see, it essentially falls in the same category.
(*Explanation for Proverbs 26:4-5? Solomon is saying that when you deal with a fool, you just can't win.)
