Yes, it's possible and what you posted should be working.
xy = myFunction(app.Panel);
If you inspect Panel component, the Spinner components will be in Children property of the Panel component. You can access each Spinner value by indexing into Children property. But, how does one know which dropdown comes first, is it the same order as in Component Browser section or the other, I am leaving you to check. You can also use the Tag property to distinguish between each Spinner.
If you want to use the values from dropdowns to do some calculations without changing panel options, maybe you should consider creating a struct that you will send to the function.
spinnerValues = struct('a', app.Spinner.Value, 'b', app.Spinner2.Value, 'c', app.Spinner3.Value)
xy = myFunction(spinnerValues);