Syntax for copyobj is two arguments, the handle (vector) of things to retrieve and the handle of the target.
I've no idea whether the syntax of f3.Children.Children.Children makes any sense at all or not, or whether that's somehow related to your having three figures, but presuming there is something that deeply nested, the (approximate) syntax would be something like
hFNew=figure; % create the target
copyobj([f3.Children.Children.Children, f2.Children, f1.Children],hFnew)
Whether that would actually work will depend entirely on just what is in the three figures f1, f2, f3 and whether those above expressions actually return something or not.
