This likely refers to the diameter of the entry holes in the junction box. A 25mm junction box would have entry holes that are 25mm in diameter.
These holes are used to bring electrical cables or conduit into the junction box.
A "three-way" junction box typically means that it has three cable entry points, allowing you to connect or branch three separate electrical circuits within the same box.
This can be useful for creating electrical connections, splices, or distributing power to multiple locations.