Dan,
My suggestion would be to use document / literal style. The data structure you describe is easy to define as an XML schema (by hand if you must, but I'd use something like XMLSpy). You can then create the requisite WSDL file referencing this schema, generate the server side Java classes against this and modify them to call the appropriate methods on your existing EJB.
If you're using doc/literal style you'll also have to build a (very simple) XSD type for your three inputs, in this case a simple sequence with minoccurs and maxoccurs attributes set to 1.
I would definitely start with WSDL in any case, given that the WSDL defines whether your service is WS-I compliant.
HTH,
Tom
