John 15 = Jesus uses a metaphor to explain how believing in him will bear the fruits of a Godly life. He instructs them to love as He has loved and tells them of the Comforter that is to come and testify of Him.
John 16 = Jesus speaks to his disciples about, among other things, His return to the Father, which He knows will happen soon. He reiterates that some tribulation is ahead for the disciples, but He also tells them to be happy and not despair, since He has overcome the world.
John 17 = Jesus prays for the Disciples and future believers before He is taken to be crucified.