The three Synoptic Gospels each record the voice from heaven using different wording.

Ehrman summarises the differences:

Mt 3:17 – “This is my one dear Son; in him I take great delight.”  The voice seems to be addressing the people around Jesus, or possibly John the Baptist.

Mk 1:11 – “You are my one dear Son; in you I take great delight.”  Here the voice addresses Jesus directly.

Lk 3:22 – “You are my one dear Son; in you I take great delight.”  This is, of course, the same of in Mark; except the Ehrman exaggerates the differences by citing the wording found in some of the older manuscripts: “You are my son, today I have begotten you”, quoting the words of Psalm 2:7.

First of all, there is a textual issue that needs to be addressed.

Bart Ehrman (Jesus, Interrupted), thinks that there is a contradiction between Matthew’s and Luke’s version:

In Matthew it says, “This is my beloved son, in whom I am well pleased.” The voice appears to be speaking to the people around Jesus, or possibly to John the Baptist, informing them who Jesus is. In Mark, however, the voice says, ‘You are my son, in whom I am well pleased.’ In this case the voice appears to be speaking directly to Jesus, telling him, or confirming to him, who he really is. In Luke, we have something different (this is a bit complicated, because different manuscripts of Luke’s Gospel give the voice different words. I am taking here the original wording of the verse as found in most English translations). Here the voice says, “You are my son, today I have begotten you” (3:22), quoting the words of Psalm 2:7).

This is wrong.  Most English translations do not have “You are my son, today I have begotten you”.  GNB, NIV, TNIV, ESV, ASV, NASB, AV, NLT, RSV, NRSV all have ‘with you I am well pleased’ or similar.  In fact, no English translation consulted by me offers anything different.  I have already noted the reason for this: the alternative reading is found in only one Gk. manuscript.

As a textual scholar himself, Ehrman should know that

‘the Western reading, “This day I have begotten thee,” which was widely current during the first three centuries, [is probably] secondary, derived from Ps 2:7’ (Metzger).

(See this, by Jonathan McLatchie)

We turn now to the other differences between the three Gospels.

Ehrman summarises the difficulty as he sees it:

Each account is trying to do something different with the voice. That is to say, the different words mean different things and have different functions: in Matthew, the words identify Jesus to John the Baptist and the crowd; in Mark, to confirm Jesus’ identity to him directly; in Luke, they declare that the baptism has made (or ratified?) him as God’s special son. But there remains the question, What did the voice actually say? Early Christians were confused by this problem, so much so that a later Gospel, called the Gospel of the Ebionites, resolved it by indicating that the voice came from heaven on three occasions. First it said the words as related by Mark, which were addressed to Jesus; then it said the words as related by Matthew, addressed to John the Baptist and the crowd; and finally the words as related by Luke. But unless someone is willing to rewrite all three Gospels, the fact is they indicate that the voice said different things.

There is, of course, no need to postulate that there were three different announcements from heaven.

France thinks that it is likely that Matthew has deliberately recast the saying into the 3rd person to match the similar saying in Mt 17:5.

Hagner suggests that Matthew has the statement in the 3rd person in order to make it more suitable as catechetical material.

‘Matthew likely presented John the Baptist’s viewpoint (see Jn 1:32–34). The voice was apparently addressed to Jesus but had the effect of confirming Jesus’ status to John. The Gospel writers did not aim at “tape recording” accuracy but at the gist of the characters’ speech. On this point see note on Mt 3:8.’

(Apologetics Study Bible, on Mk 1:11)

Ramsey Michaels (Servant and Son) notes that Matthew

preserves the singular verb ‘he [Jesus] saw’ (v16); the dove was visible to Jesus, and not (presumably) to the bystanders;

the third-person form of the announcement from heaven is, accordingly, likely to be a testimony to the reader, rather than an indication of what any bystander heard.