In the Hebrew canonical arrangement the former prophets are Joshua, Judges, Samuel, and Kings and the latter prophets are Isaiah, Jeremiah, Ezekiel, and the Book of the Twelve. The number pattern of 3+1 and 3+1 is a feature of Biblical wisdom literature. Usually the +1 is the punch — Kings ends with the nation of Judah in exile; the Twelve ends with Malachi announcing the coming Day of YHWH. I would dismiss this shape as simply coincidental, understanding that word in a context of faith, except that we have seen that the structure of the Writings is purposeful and because the Twelve were stitched together to make the pattern work. Should you wonder what the purpose of a wisdom shape to the prophets might suggest — it is quite simple, study, study, study the written words of the prophets. It is an apologetic for the importance of the study of the written word of God.
It is no surprise, as I have observed elsewhere, that the prophets begin and end with wisdom language that emphasizes studying the Torah. The prophets are therefore styled as wisdom teachers of the written Torah. Joshua says, “Only be strong and very courageous, being careful to do according to all the law that Moses my servant commanded you. Do not turn from it to the right hand or to the left, that you may have good success wherever you go. This Book of the Torah shall not depart from your mouth, but you shall meditate on it day and night, so that you may be careful to do according to all that is written in it. For then you will make your way prosperous, and then you will have good success” (Josh 1:7-8, ESV modified).
Likewise, the ending of Malachi resounds the wisdom theme of studying the written word: “Remember the Torah of my servant Moses, the statutes and rules that I commanded him at Horeb for all Israel” (4:4 in English, ESV modified). Thus the way that the prophets are arranged highlights the way people are to use the prophets until the Day came — study the written word (and in particular: study them as wisdom teachers of Deuteronomy). One should even wonder if the order throughout Joshua 1 of “Moses, my servant” is purposefully switched for Malachi 4:4 to “my servant Moses.” The stress being that the written word of God is God’s servant.
It is worth noting that one of the effects of putting the Book of the Twelve last is to give it more importance. It is understandable that particularly Isaiah would be perhaps the most influential book of the latter prophets because of its length. The Twelve gets some extra weight, which is lost in the English Bibles, by being read as a single long book. Moreover, it is the +1 book. And even furthermore it is the final book of the Prophets. All of these considerations promote studying the Twelve more than they often are. But like the Writings, sometimes cited by the first book the Psalms, citing Isaiah (or occasionally Jeremiah) is often shorthand for citing the latter Prophets in general.
The Book of the Twelve was stitched together and it follows a discernible geographical pattern — north (Hosea to Israel), south (Joel to Judah), north (Amos primarily to Israel), south (Obadiah to Edom), north (Jonah to Ninevah), south (Micah to Judah), north (Nahum to Ninevah), south (Habakkuk to Judah), south (Zephaniah to Judah). Then the last three after the restoration of Jerusalem has begun (Haggai, Zechariah, and Malachi) are concerned with priestly things, which mirrors the emphasis on priests in the wisdom tradition since they taught Torah. The latter Prophets moves in this direction as a whole in that the last two books are Ezekiel and the Book of the Twelve. But the point here is that the Book of the Twelve has a purposeful order like drawing a spiral in to Jerusalem. It should be no surprise then that the gospel reverses this by going from Jerusalem, to Judea, to Samaria and the ends of the earth.
By the way, in the Hebrew Bible the scribes saw the Book of the Twelve as one whole book and made it clear by putting only three spaces instead of four between each one and by giving statistics on the whole.
So we have 3+1 and 3+1. The hero of Kings (the +1 of the former prophets) will return before the coming of the Day (as noted in the +1 of the latter prophets: Malachi 4:5-6, English). Until then, study the written word of these wisdom teachers of Torah. And now that this Elijah has come as John the Baptist and the +1 (eighth) day has begun with the death and resurrection of Jesus Christ, continue to study the written word until the seventh day is done.