The book of Acts recounts the spread of the early church and the manner in which Christ works to take the gospel from Jerusalem, to Samaria, and finally to the ends of the world (Acts 1:8). In your third sermon you must focus on the coming of the Holy Spirit at Pentecost. It will be helpful to provide a summary of what exactly happened, since many people have strange ideas when they read of fire etc. You should also spend time explaining why the Spirit came specifically at Pentecost, after Jesus’ ascension, and how his coming is different from his work in the old covenant. The goal of your sermon is thus to help God’s people rightly understand the significance of the coming of the Spirit.