An approach is the philosophical or theoretical justification of certain field of knowledge e.g. language. Empiricism and rationalism define language in their own philosophical terms.
Method, on the other hand, is the statement of how the content of the language should be presented under the given theoretical assumptions. What aspects of the language to select for classroom purpose, how to present them in the syllabus and how tho present the selected topics in the classroom are well stated under method. It is procedural and more precise than the approach.
Technique, on the other hand, is implementational. It is the set of activities and materials.