Well you can’t. Even if you used a Container to produce that, you still cannot do it, unless you assigned each part of the button or container the same action.
You can’t (yet) assign actions for arbitrary events when a mouse hovers over a button. At most you can use an Active{Icon,Title.Colorset}.
Okay, it’s not really a problem to assign the same action to all three buttons.
But I didn’t understand your last sentence. Is it possible to mark all three buttons (or change their background color) when i just hover one of them?
Yes – look at the ActiveColorset command in ‘man FvwmButtons’ – assuming you’re using FVWM 2.5.X. I will look at patching FvwmButtons to allow for the request you asked for initially. Stand by.