Plan: A detailed proposal for doing or achieving something. A plan for an activity must be based on some idea of a method of work.
Method: A method relates to a type of activity in general. The output of one method might be the input to another. In other words, a method refers to a single action, tool, technique, process, or way of doing something.
Methodology: Groups of methods or techniques are often grouped into methodologies such as object-oriented design. A body of methods, rules, and postulates employed by a discipline; a particular procedure or set of procedures. Essentially, a methodology is a collection of methods, practices, processes, techniques, procedures, and rules.