How to disable buttons in GTK3

For specific control, I think you can use "set_sensitive":

var button = new Gtk.Button.with_label("Hello");

button.set_sensitive(false); // true to enable the button

gtk_widget_set_sensitive (widget, FALSE);

Note that if you need to check whether a widget is sensitive, gtk_widget_get_sensitive() returns the value set with the above function and gtk_widget_is_sensitive() will tell you whether the widget really is sensitive -- this is affected by parent widgets sensitivity as well.