Hej Mats - welcome to OpenHab
I can answer only 50% of your question, as I do not have migrated to Openhab 3.0 myself. However the principal setup of a connection of the Openhab and Controme system stays the same. Ido not know, what your knowledge about the controme-OpenHab connection is - so please excuse, if all of this is well known to you.
- transfer room temperature data from Netamo and Homee to controme
Here you need to obtain one virtual sensor per temperature value you want to send from openhab to contome. These virtual sensors are sold in the Controme webshop for app 10€ each and are in the end of the day solely a sensor address in the controme controller, which you receive from Controme.
A rule in OpenHab can than send an updated temp value from your netatmo or Homme inputs to Controme. To my knowledge OpenHab 3 has different rule engines. The rules in this thread are using the “old” system which is based on a kind of Java language. A pointer might be to understand how this legacy rule engine works in Openhab 3.
- visualize and steer the controme system by open hab
The script by @BoBiene does exactly this. If you know the addresses of the Controme sensors this is quite straightforward. at least in OpenHab 2.5 - for Openhab3 there might be some caveats with the different rule engines (see answer above).
I hope this helps you and you can feedback here on the forum about further questions and successes. I will most probably migrate to openHab3.0 soon as well - and hope all will work out with the Controme connection.