There has been no definitive origin to the Triple Changers. In Dreamwave's Generation 1 Universe it is said that Soundwave created them.
List of known G1 Triple Changers
In Beast Wars, the term Triple Changer was replaced with Three Mode Conversion
Only two characters are mentioned as Triple Changers, yet the Transformers: Armada More Than Meets The Eye profile comics state than all Mini-Cons with a weapon mode are Triple Changers.