The lectionary gospel reading for Advent 3 in Year A is Matt 11.2–11, and as usual is rather truncated, so you might want to extend the reading by a verse or two before and after to be fair to its setting.
John the Baptist appears to either question or express disappointment in the ministry of Jesus, and the passage then moves on to related sayings about John.
James and Ian discussion the passage, its context, and the issues it raises for our discipleship and our preaching on this passage.