Season 2 of the G1 Transformers show introduced the Triple Changers, who had two alt-modes in addition to their robot form. Unlike the Combiners, they were introduced with no fanfare, and just showed up as if they had been among the Decepticon ranks the whole time (Autobot Triple Changers would show up later).
What makes the Triple Changers so different that they can have multiple alt-modes when the vast majority of Transformers cannot? I'm primarily interested in the G1 continuity, but will accept answers from later canons if nothing was ever explained.
*Note: This is not a duplicate of Why do the Transformers only become one type of vehicle?, which is similar, but asks why TF's don't change alt-modes, and is focused on the live-action movies.