Matthew 18 describes a graduated process when one disciple sins against another: private confrontation, then the involvement of one or two others, and finally the community. The presence of additional people is not merely a strategy for applying social pressure.
The wording recalls Israel's legal principle that a matter should be established by two or three witnesses.
Deuteronomy requires multiple witnesses for serious judicial matters, guarding against condemnation on the unsupported word of a single accuser. Jewish legal and communal life inherited the importance of corroborated testimony.
Matthew adapts that scriptural principle to the life of Jesus' community. The additional persons can help establish what has occurred and move the dispute beyond one person's allegation against another.
The legal background makes the procedure look less like an escalating campaign to shame an offender. It builds deliberation and corroboration into the process before the matter reaches the wider community.
That fits the chapter's larger concern with restoration, forgiveness, and care for members of the community. Accountability is serious, but it is not supposed to rest on unsupported accusation or impulsive public exposure.