A: Lets take the last question first. "It becometh us to fulfill all righteousness" means, slightly paraphrased, It is appropriate [or, proper] for us to do this, because it is the right thing to do." Here the KJV uses the term "becometh" in a somewhat archaic sense, akin to the phrase, "That lavender dress becomes her" (meaning that the wearer looks comely in the dress).
To answer the other two questions helpfully its a good idea to first give some idea of the background of what John the Baptist was doing. It was not a new idea to wash in water as a step in presenting oneself to God for His service. The priests in the tabernacle and Temple did that (cf. Numbers 19:7). Ex-lepers, in the process of having their leprosy be pronounced cleansed, did that (cf. Leviticus 14:1-9). Luke mentions that Jesus began His ministry when He was about 30 years old (in Lk. 3:23), and this was the age at which apprentice-priests began their official service-period (cf. Numbers 4:46-47. This also happens to be the age of David when he began to rule; cf. II Samuel 5:4). And, the community which produced/collected the Dead Sea Scrolls (lets call them Essenes) seems to have immersed themselves daily in baptistry-like pools called miqveh.
(Also, Gentile males who converted to Judaism were circumcised and took part in ritual immersions as a rite of passage. The washing in this case symbolically added the individual Gentile to those who, like the rest of Israel, passed through the Red Sea in the days of Moses, and thus became heirs of Gods promises as His chosen people. Pauls choice of words in First Corinthians 10:2 is probably connected to this motif, btw. And, it may be that the religious leaders became curious about John because of this use of baptism. To them, John was indirectly telling people who were already descendants of Abraham that they needed to become descendants of Abraham. John seems to anticipate this sort of question in Matthew 3:9.)
All those ideas -- baptism as a ritual signifying purification, self-dedication, repentance, and entry into Gods chosen nation -- were wrapped up in what John was doing. That is why he was confused to see Jesus coming to be baptized: baptizing a non-sinner would be a bit like giving the proclamation No More Leprosy to someone who had never been a leper.
Nevertheless Jesus insisted on being baptized. Although other aspects of baptism didnt apply to Jesus, the aspect of self-dedication for the service of God did. And that is why Jesus was baptized. His baptism was unique; it was a sort of coronation (in His kingly role) and dedication (in His priestly role). His baptism also serves as an example for all who want to do the Fathers will -- all of us who, under the new covenant of grace, are offered baptism in the name of the Father, and the Son, and the Holy Spirit, for the forgiveness of sins.
Yours in Christ,
Waterrock
