Hard to say what they were showing in Holland at the time. It sounds along the lines of vehicle Voltron, but with only 3 vehicles. There was a toyline called 'Abega' that sounds similar. You may want to check that out.
There were literaly tons of transforming/combining robot lines in the 80's. If you can't find anything on the cartoon, there was most likely a toyline for the show. You may want to try searching through some sites like toyarchive.com or virtualtoychest.com and going from there.
I'm not quite sure, but after reading your description the old cartoon "Orbots" comes to mind. I googled it and found this site ( [url]http://well-of-souls.com/orbots/orbots.htm[/url] ). Is this what you were thinking of? If not, thanks anyway for jogging my memory about the Orbots. I loved this show as a kid!
Show-wise, it's probably Getter Robo. Toy-wise, it's either that or Voltron 3, which was never shown in the US, but toys were made anyway.
I like to think this is Getter Robo as well, for which an US dub based on it's sequel "Getter Robo G" was aired under the Force Five title "Starvengers" in some markets around 1980, as well as a subsequent releaseon video and shown on Showtime in a feature-length compilation.
Here's a nice treat, the opening to "Getter Robo G" as seen from an Los Angeles station some 25 years ago. [url]http://www.youtube.com/?v=yNlj6LX_LkU[/url]