"Three Days And Three Nights" Reconsidered

This book explains why the author could no longer in good conscience hold the Wednesday/Saturday view concerning Christ's burial and resurrection. Why does only ONE verse say "three days and thee nights," and 20 verses say "the third day" or "in three days"? Figuring that a day ends at sundown, how should we explain the wording: "in the end of the sabbath, as it began to dawn..."? Was a time period the "only" sign Jesus gave that he was the Messiah?