Why was Jesus baptized? (Matt3:13-17; Mark 1:9-11;John 1;32-34) Was he baptized so all would know that he is the Son of God?
First of all, not all of the verses that you cite in this question actually show Jesus being baptized. All of the Synoptic Gospels do tell the story of Jesus’s baptism (you left out Luke 3), but John does not tell this story.
Second, we are never explicitly told why Jesus was baptized. None of the gospels gives us a clear reason for this. In Matthew, we are told that it is “to fulfill all righteousness,” but we are not really told what this means. Therefore, we are left to reason it out as best we can.
There are a number of possible reasons that it might have been important for Jesus to be baptized. They include:
Announcing him as the Son of God, as you mention. It might be that this was the best way, in God’s eyes, to formally announce who Jesus was at the start of his ministry.
To show Jesus’ formally accepting his ministry and being obedient to God. This is one meaning that is often given to the phrase about fulfilling “all righteousness.” It is sometimes read to mean that Jesus is simply obeying God’s command. If so, then this baptism marks a point when Jesus openly accepts God’s command to him that he should embark on this public ministry that will result in Jesus’ death. Jesus is baptized to show that he obeys God’s command and accepts the task that is given to him.
To identify himself as closely with human beings as possible. It could be that Jesus needed to be baptized so that he could fully experience what people experience. Jesus’ mission was to live and die as a human being (who was also divine) so as to atone for our sins. To do so, he may have needed to be baptized because that is what humans do.
We cannot know for sure why Jesus was baptized. We can only speculate because the authors of the gospels did not give us an explicit explanation for this event.