This passage occurs in the RCL Epiphany Season, Year C, most recently January 2013.
Summary: I get why the lectionary dismisses vs 18-20. However, I would encourage you to add them back in. John ended up in prison; all those who come near the waters of Baptism risk their health and life. This is perhaps why Baptism for Luke is so tied to prayer -- because where there is Baptism, there is the cross, and where there is the cross, there will be prayer. I also recognize why the lectionary separates out Jesus Baptism from Jesus' temptation. But again, this is highly problematic because it robs Baptism of its fundamental character: entrance into the Spiritual warfare of Christ against all evil in the world including in ourselves.
Three sermon ideas based on the Greek:
What are you waiting for?
3:15 Luke here uses the word, "prosdoka-oo" for "wait" or "expect." Interestingly, Luke uses this word a whole bunch (6x in Luke; 4x in Acts), far more often than anyone else. In this case though, the people are not waiting for Jesus, per se, but rather the Messiah, and wondering whether John would be it. Perhaps a reminder and a challenge -- what are we waiting for? Jesus shows up when we were expecting something and offers us REAL life.
Power of prayer:
3:21 Once again the Gospel of Luke, we find Jesus praying. The word "praying" is a present participle in this case, which means it is a concurrent action. The question of course, if which verb is it concurrent with: the Baptism or the opening of the heavens? The Greek here presents a grammatical ambiguity; perhaps it alludes to a spiritual mystery. Its intersection points toward another insight: Prayer is what unlocks the power of our Baptism. God has claimed us and established a relationship with us. Prayer is how we live into this relationship -- how the heavens are opened to us.
(I would add that the grammar leans toward the pray being concurrent with the heavens opening. Regardless, the first action after Jesus' Baptism is prayer.)
The word baptize is used four times in a few verses here. I think Luke wants to draw our attention to the actual action. Perhaps to tie it back to prayer, because of the act of Baptism, we always hear the answer to our own prayers: That we are a beloved child of God and brother of Jesus Christ, claimed in the waters.
Incarnation of the Spirit:
3:22 At Christmas we celebrate the incarnation of the flesh; in Baptism we celebrate the incarnation of the Spirit! The Holy Spirit fleshed itself -- it came "soma" (body) style! The Spirit again become flesh in our Baptism into the body of Christ.