In the structure of the gospel
I assume that James, John and Peter, the three pillars of the Jerusalem church from Galatians 2:9, denote three original Christian groups with their own traditions.
Mark puts these different traditions together for the first time.
According to 9:5, Peter wants to erect three temple tents for the law (Moses), prophecy (Elijah) and the new covenant (Jesus).
The three temple tents are symbols for the original Christian currents Jewish Christians (James), Baptist Christians (John) and Spiritual Christians (Peter, Christian gnosis).