Many writers prefer to place the thesis statement or hypothesis here, which is perfectly acceptable, but most include it in the last sentences of the introduction, to give the reader a fuller picture.
The idea is that somebody will be able to gain an overall view of the paper without needing to read the whole thing.
The third part should give the reader a quick summary of the form that the parts of the research paper is going to take and should include a condensed version of the discussion.
This should be the easiest part of the paper to write, as it is a run-down of the exact design and methodology used to perform the research.
Obviously, the exact methodology varies depending upon the exact field and type of experiment.
There is a big methodological difference between the apparatus based research of the physical sciences and the methods and observation methods of social sciences.
For quantitative research, it is a presentation of the numerical results and data, whereas for qualitative research it should be a broader discussion of trends, without going into too much detail.
For research generating a lot of results, then it is better to include tables or graphs of the analyzed data and leave the raw data in the appendix, so that a researcher can follow up and check your calculations.
This is where you elaborate on your findings, and explain what you found, adding your own personal interpretations.
Ideally, you should link the discussion back to the introduction, addressing each point individually.