So, who was John baptizing? Jews. His baptism was called the baptism of repentance. However, he also says that Jesus, the one who would come after him and mightier than he, was going to baptize them also, not with water, but with the Holy Ghost and with Fire. That begs the question, "When were they to be baptized with the Holy Ghost?" Well, that happened at Pentecost for those who believed. Well, "When were they to be baptized with fire?" The context makes it clear that the fire baptism that is was not a good thing and it is not the "fire" experience that many charismatics talk about today. Notice the colon at the end of verse 11? They are used before explanations or reasons. Therefore, verse 12 gives the explanation of the reason for this fire baptism. It is obviously about a judgment that was to come after the nation had repented, i.e. Tribulation.
There is a fourth baptism mentioned in Luke 12:49-50 where our Lord said, "I am come to send fire on the earth; and what will I, if it be already kindled? (50) But I have a baptism to be baptized with; and how am I straitened till it be accomplished!" This was a reference to when God the Father baptized his Son with the sins of the world on the cross.