"So Jonah arose, and went unto Nineveh, according to the word of the LORD. Now Nineveh was an exceeding great city of three days' journey. [And Jonah began to enter into the city a day's journey]" LXX: 'so Jonah arose and went to Nineveh, according to the word of the Lord. Now Nineveh was a city of godly size, around three days in journey. Jonah began to enter the city, about one day's travel.' Jonah immediately carries out the command that he has been given. Nineveh to which the prophet was journeying, was a great city, which it took around three days' journey to circle. But he remembers the command he has been given and the recent shipwreck and makes the normal journey of three days in one day. However, there are some people who believe that he simply proclaimed his message in a third of the city, and that his speech quickly was made known to the other inhabitants. And our Lord is said to arise and speak of his own accord after being in hell, and announces the word of the Lord when he sends the apostles to baptise those who were in Nineveh in the name of the Father the Son and the Holy Spirit [Mt. 28:19]. So there are the three days of journey! And this sacrament of mankind's safety is "a journey of one day", that is it is finished by the proclamation of one sole God. Jonah preaches not so much to the apostles but more by the method of the apostles. He himself says, "and I will be with you always until the end of the world" [Mt. 28:20]. There is no doubt that Nineveh was a city of godly magnitude because the world and all things have existed through God and because without Him nothing would ever have existed. [John 1:3] Note too that he has not said, "of three days and three nights" or "of one day and of one night", but simply "and of three days", and "of one day", to show that in the sacrament of the Trinity and of the confession of one sole God there is no darkness.
Why, then, are we asked what was prefigured by the prophet being swallowed by that monster and restored alive on the third day? Christ explained it when he said an evil and adulterous generation seeks a sign, and a sign shall not be given to it, but the sign of Jonah the prophet. For as Jonah was in the whale’s belly three days and three nights, so shall the Son of man be in the heart of the earth three days and three nights. … So then, as Jonah went from the ship into the belly of the whale, so Christ went from the tree into the tomb, or into the abyss of death. And as Jonah was sacrificed for those endangered by the storm, so Christ was offered for those who are drowning in the storm of this world. And as Jonah was first commanded to preach to the Ninevites but his prophecy did not come to them until after the whale had vomited him out, so the prophecy made to the Gentiles did not come to them until after the resurrection of Christ.
[AD 420] Jerome on Jonah 3:3