Textual: The chapter break at this point obscures the narrative flow of this section. It is helpful to restore some of the context to understand what Mormon is doing at this point.
3 Nephi 26:17
17 And it came to pass that the disciples whom Jesus had chosen began from that time forth to baptize and to teach as many as did come unto them; and as many as were baptized in the name of Jesus were filled with the Holy Ghost.
3 Nephi 26:18
18 And many of them saw and heard unspeakable things, which are not lawful to be written.
3 Nephi 26:19
19 And they taught, and did minister one to another; and they had all things common
among them, every man dealing justly, one with another.
3 Nephi 26:20
20 And it came to pass that they did do all things even as Jesus had commanded them.
3 Nephi 26:21
21 And they who were baptized in the name of Jesus were called the church of Christ.
3 Nephi 27:1
1 And it came to pass that as the disciples of Jesus were journeying and were preaching the things which they had both heard and seen, and were baptizing in the name of Jesus, it came to pass that the disciples were gathered together and were united in mighty prayer and fasting.
The chapter division was made under the assumption that the “and it came to pass” of 3 Nephi 26:17 indicates a different incident from 3 Nephi 27:1 where we have “and it came to pass” again. It is true that this is typically a marker of the passage of time and events, but Mormon’s narrative at this point it time is not strictly chronological. In the sample text above we have from 3 Nephi 26:17-21 as a generic text that covers an undetermined time period. That text ends with the information that the church is “called the church of Christ. What Mormon has done is give an overview of the history, and will now give a specific instance. This is the same process we say in 3 Nephi 26:14-15 where the generic overview preceded the specific detail.
In this case we have the overview picture of what the apostles were doing, ending in the baptism into the church which was “called the church of Christ.” In this specific case, Mormon will now give us the details of how the church came to have that name. This specific instance begins by paralleling the introduction from 3 Nephi 26:17. There the disciples are teaching and baptizing. In verse 1 of this chapter, the disciples are preaching and baptizing. The additional information is that on this very occasion they are engaged in prayer.