The beginning of the third day followed the model of the second day. The twelve taught the doctrine of Christ: repentance and baptism. After having their God physically present with them for two days, it is easy to imagine that the people were both teachable and anxious to accept the doctrine of Christ. Therefore, even more were baptized.
Less clear in these verses is a shift between verses 16 and 17. In verse 16, Mormon is describing the third day’s visit; then he stops.
In verse 17, we pick up on the aftermath of Christ’s visit. The apostles continued to preach and to baptize. This is not the end of a chapter in the 1830 edition, and is a mildly unfortunate division, because 3 Nephi 27:1–22, which originally ended this chapter, really is part of the discussion of the twelve after Christ ascended after the third day. If there were to have been an artificial division, it would have been better to have begun at verse 17 of this chapter in order to keep the story of the twelve together.