I set aside the first two Chapters of Matthew as being unreliable, probably legendary accretions and/or adopted to butress the argument of the Gospel as a whole. So Chapter 3 is the first part of Matthew's Gospel that is consistent with the Fourth Gospel - and it is an account of the teachings of John the Baptist.
This is interesting, as the author of the Fourth Gospel was a disciple of John's before he became a disciple of Jesus - so when there is subtanative disagreement, the Fourth Gospel is clearly to be preferred.
Let's see whether there is anything about Matthew 3 that adds to or deepens our understanding of John.
 In those days came John the Baptist, preaching in the wilderness of Judaea,
 And saying, Repent ye: for the kingdom of heaven is at hand.
 For this is he that was spoken of by the prophet Esaias, saying, The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
 And the same John had his raiment of camel's hair, and a leathern girdle about his loins; and his meat was locusts and wild honey.
So far this is much the same as the Fourth. Except for: Repent ye: for the kingdom of heaven is at hand. Here Matthew seems to be reporting John stating that there is not much time for those living 'now' to repent. He will later have Jesus say the same.
 Then went out to him Jerusalem, and all Judaea, and all the region round about Jordan,
 And were baptized of him in Jordan, confessing their sins.
 But when he saw many of the Pharisees and Sadducees come to his baptism, he said unto them, O generation of vipers, who hath warned you to flee from the wrath to come?
 Bring forth therefore fruits meet for repentance:
Matthew introduces here a favourite 'repent or else' theme (not a part of the Fourth Gospel) that there is a wrath to come, an apocalypse, the second coming of Jesus; which will specifically affect the Pharisees and Sadducees to whom John is talking. In other words the wrath is coming soon. The implication seems to be that John's baptism was a matter of people confessing and repenting their sins and being absolved; and that this procedure was to safeguard these people from the wrath to come.
The attack on Pharisees and Sadducees who are coming to John for Baptism seems wrong. Surely they are doing precisely what Matthew wants everybody to do - which is, from terror of the burning to come for those who are not baptised, they will accept baptist at the cost of repentance.
Matthew's basic method in his Gospel is to induce fear of 'Hell' which will be inflicted soon; and describe what to do to escape this fate. What to do entails following a set of rules that is even more numerous and rigorous than those of the Pharisees - Jesus adds to The Law. But this was presumably regarded as a genuine possibility precisely because the apocalyptic second coming was not far away...
 And think not to say within yourselves, We have Abraham to our father: for I say unto you, that God is able of these stones to raise up children unto Abraham.
 And now also the axe is laid unto the root of the trees: therefore every tree which bringeth not forth good fruit is hewn down, and cast into the fire.
This is the 'or else' part of of the 'repent, or else'. In other words, if people do not confess, repent and be baptized - they will be 'cast into the fire'. I suppose that the attack of the Ps and Ss is actually a criticism of their lack of rigour, their hypocritical failure to live by the laws they profess to follow. Whereas in the Fourth Gospel, Jesus preaches an extremely simple message of loving and following Jesus to life everlasting; nothing about following laws or living a prescribed life.
 I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:
 Whose fan is in his hand, and he will throughly purge his floor, and gather his wheat into the garner; but he will burn up the chaff with unquenchable fire.
Here is the idea of 'Hell' as an eternal punishment, of unquenchable fire; which will be implemented soon - within the lives of the people to whom John is talking. And also the idea that Hell is the default state - which everyone will go to excepting those who are saved. Jesus will divide people into those he gathers, and those he casts into fire.
Then cometh Jesus from Galilee to Jordan unto John, to be baptized of him.
 But John forbad him, saying, I have need to be baptized of thee, and comest thou to me?
 And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him.
 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:
 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
This is descriptively similar to the account in the Fourth Gospel - but whereas in the Fourth this event is a divinization of Jesus, leading directly on to his miracles; no such link is made by Matthew.
In sum, what we get from Matthew Chapter 3 (compared with the Fourth Gospel) is the completely different, alien and contradicting idea that the main thing about Jesus was that he was shortly to return, and divide Men into his own whom he gathers, and those who shall burn forever.
This teaching is also attributed to John the Baptist, and the only difference is that John was an agent describing how to be saved from the burning; while Jesus is the judge who will himslef make the division of men. John gave the theory; Jesus will implement the practice. So this is the implied sense in which John 'prepares the way of the Lord'.
By contrast the Fourth Gospel has nothing about this scheme; no imminent second coming, no burning punishment; and John the Baptist's role seems to be to enable Jesus to become divine by baptism.
In Matthew, however, Jesus is already (in Chapters 1 and 2) marked-out as divine by his miraculous, prophecy-fulfilling childhood and youth; so John's baptism is merely done in order to 'fulfil all righteousness' and to enable the signs of the descent of the dove and the voice from heaven.
So, do I get anything valuable from Matthew 3? Just one thing - which is that what John was doing with his baptising was probably a temporary 'cleansing' of the individual from sin; active only in this mortal life and with no implications for eternal life.
What about "he shall baptize you with the Holy Ghost, and with fire" - this suggests that Jesus will himself be doing a better kind of baptism. But the Fourth Gospel states that Jesus himself never baptised anyone. So this is either an error, or else refers to baptism in a symbolic way.
Since Jesus is assumed to be returning imminently to dive the heavenly wheat from the hell-bound chaff, it looks as if this means the judgment itself: baptism of the Holy Ghost is for the wheat; and baptism of fire is for the chaff, baptism of fire being an indirect way of referring to the unquenchable fires of damnation.
This corresponds with John's report in the Fourth Gospel that the 'normal' thing that happened when he baptised, was that individuals were 'touched' by the divine spirit; that they were momentarily divinized and thereby (presumably) cleansed of their sins up to that moment. John did not change the prospect for the individuals he baptised, he did not change their fate - but, nonetheless, what John was doing (as described in the Fourth Gospel) was new, different, and contrary to the usual laws and rules and sacrifices of the Jews of that era. John was, in this sense, the first to practise the new dispensation; albeit in a partial, and this-worldly, form.
But when Jesus was baptised by John, uniquely the spirit stayed upon him, meaning (I suppose) that Jesus became permanently divine by the agency of John: John had this absolutely vital role (as the Fourth Gospel implies) in making Jesus fully divine.
Note: I found the above detailed analysis to be a depressing exercise; and probably will not continue with it through the rest of the Matthew Gospel. It is the kind of thing that becomes necessary when the Fourth Gospel is really believed - but it is probably better to make such decisions in a broad brush way than to engage in this kind of miserable dissection...