Thanks all, I know the libdazzle library and the DzlMultiPaned widget,
as a simple layout container widget, I just think GtkPaned widget could be stronger.
Nobody is working on making GtkPaned support multiple panes/child, and it’s unlikely it’ll be changed before GTK4 is released. The API reflects the fact that it is a two-children container, so it’s not a case of simply adding support for additional children.