It's interesting that the parallel account in Luke didn't mention the three days as the sign of Jonah.
When the crowds were massing together, he began to say: “This generation is a wicked generation; it looks for a sign, but no sign will be given to it except the sign of Joʹnah. 30 For just as Joʹnah became a sign to the Ninʹe·vites, so will the Son of man be to this generation. 31 The queen of the south will be raised up in the judgment with the men of this generation and will condemn them, for she came from the ends of the earth to hear the wisdom of Solʹo·mon. But look! something more than Solʹo·mon is here. 32 The men of Ninʹe·veh will rise in the judgment with this generation and will condemn it, because they repented at what Joʹnah preached. But look! something more than Joʹnah is here. - Luke 11:29-32
I wonder if the Ninevites even knew Jonah was in the fish for three days.