Definition of a disciple is someone who follows Jesus.
A disciple is someone who seeks first the Kingdom of God and His Righteousness, which involves Kingdom work.
The words of Jesus have authority.
The first man: we should consider all the costs, in case future costs make us turn back
The second man: has not considered the urgency; we will always have desires that we place before the Kingdom, but the Kingdom comes first.
The third man: the man refers to Elijah allowing Elisha to say goodbye, but God says 'listen to my Son'. Not very interpretation of scripture is valid, and not every valid interpretation is equal.
Following Jesus means involvement in Kingdom work.
Jesus has the right to order our priorites.
Jesus and Kingdom work overshadow other work; eternal takes precedence over temporal.
Consider the cost of not following: value of Jesus, urgency, and what He has come to do.