I have installed an Intergas Gateway V3.
It works fine on the iPhone and iPad.
When I try to install it on the Homey, I get the message “pairsession_not_found”
This happens when you switch apps on your phone, for example to get your password from a password manager. When you switch apps, the pair session expires. Make sure you have everything you need on your clipboard before you start the pairing
Plesse find yourself a v2 model to connect it to Homey.
In the meantime, someone reverse-engineered the Intergas phone app to get the v3 GW to work with Home Assistant using the Intergas cloud connection.
So, alternatively, an easy (and quickest) way is to add Home Assistant to the mix.
You can request for Homey support per Intergas Incomfort app topic,
or maybe @smarthomesven is seeing possibilities for a separate Intergas GW v3 app.