I seem to have a similar issue as the user @trololirko (post-link) and @Posse
I bought a simple 3 way switch from a local hardware store:
And I connected it in this way, with a Yellow, Red and Blue cable, to the Multiplus 12/800/35
I guess we do it like this, right?
This whole Q&A thread was EDITED by Guy Stewart to add clarity to the answers.