The III divisioona was introduced in 1973 and in the mid-1990s became known as the Kolmonen (Number Three in English and Trean in Swedish).

Until the end of 2023 season, Kolmonen was the fourth level in the pyramid, but after the creation of new second-tier Ykkösliiga, Kolmonen fell down one level, starting in 2024.

There are 108 clubs in the Kolmonen, divided in 10 groups of 8 to 12 teams each representing a geographical area.

During the course of a season (starting in April and ending in October) each club plays the others twice, once on their home ground and once on that of their opponents.

Since 1996 until 2020 the administration of the Kolmonen was delegated to the 12 district authorities of the SPL.