A prenup that you first saw 3 days before the wedding would typically be considered to be signed under duress, and a good lawyer could invalidate it in court. Plenty of precedent there.
"A prenup could end up being useless if one of the spouses signs it against his or her will. Why? If that spouse contests the prenup, the court could void it.
"In order for the prenup to be valid, both parties need to enter into it freely and voluntarily," says Nihara Choudhri, author of "What to Do Before the I Do: The Modern Couple's Guide to Marriage, Money and Pre-nups."
That's one reason why the partners should start talking about a prenup well before their wedding day. "It's much better to have it done two months or a month before the wedding than two weeks or a week before," says Frederic Siegel, a divorce attorney with Fitzmaurice & Siegel in Stanford, Conn. "Then you eliminate the argument that one of the spouses was under duress or they didn't have time to consider it." In fact, he adds, these days some couples actually videotape the signing of the document, so if the prenup is challenged, there's evidence that both sides knew what they were doing."