Did you realise help is only available when the plugin is selected?
Yes, and in that case the button is in-active.
Also only if the selected plugin implements it.
Yes, I know that not all plugins have implemented the help feature. I tried more than one and also re-tried on one which worked on gtk2.
But here is an update: I just found out that it only happens if I run geany as root. If I work with a normal user login under gtk3 then it works like on gtk2. So I assume this can be closed as it might be caused by some file on my machine having the wrong permission?