So I think what you're saying is that the data connection is returning one row with field1, field2, field3. And what you want is a dropdown in the form that shows these three as the options within the dropdown?
Again, if all you want is:
Ghoztrider:a filtered text field to populate with the data from a field in the parent item
Then I still don't understand the need for a dropdown. Some simple rules will allow you to set the value of a field in the main data source.
(Why are you using a web service, by the way? Isn't this available out of the box in InfoPath by setting the QueryFields before calling the data connection?)