Do I need "Introduction" and "Conclusions" parts in my PhD thesis?

Run, don't walk, towards the advice of your advisor.

I'm not sure why you are questioning their "experience writing a thesis in parts."

You should add a broad introduction and broad conclusion that service your entire thesis; these will be before and after your Part 1 and Part 3, respectively.

You should add individual introductions and conclusions to each Part. Therefore you will have a total of 4 introductions and 4 conclusions, with 3 of each that have a narrower scope and 1 of each that have a broad scope.

You will probably additionally have introductions and conclusions within each chapter in each part, though you may not label them as such and they could simply be paragraphs. This is simply good writing and is not at all related to thesis writing specifically.

Your adviser's suggestion is a tad unconventional based on my individual experience (which may be institute and field-specific), but is not untenable. It may actually be very good advice.

As goes the common joke, while there would be a common theme which ties up your entire thesis in unison, perhaps the three parts you describe might pertain to a common methodology/approach being utilized in three altogether different contexts. If this is true, this is not at all an unusual, and under these circumstances, you have been given very good advice. Three different contexts each deserve their own respective introduction and conclusion sections.

In this case, I would offer you the following advice:

• If you are uncomfortable having 4 different "Introduction" sections in your thesis, you can perhaps try this. Call the first introduction section "Introduction" itself, and make it a general introduction to the common theme, i.e. the approach you pursue in this work. While you tease the contexts A, B and C, do not go full throttle describing them here in this section. Just mention them, and say, they will be elaborated on later in the thesis.

• The individual introductions to the three parts need not be named "Introduction". (Though if you want to call it by that name, your wish.) You can give it some other alternative title, such as "Prelude", or nothing at all. Latter is also a very reasonable choice, an untitled 2-3 page monograph at the beginning of a new section will be automatically understood as an introduction only. What else can it possibly mean?

• Likewise, for the three individual conclusions sections, and the final overall conclusion - they will have different composition, even if you choose to give them the same name. The individual conclusion sections should conclude the findings of the individual parts, while the final conclusion section would basically describe how magnificently the common theme/approach worked in altogether different contexts to elucidate how nature works as per the same laws etc. etc. This final conclusion section should only reflect over the detailed individual parts' conclusions, not go through them comprehensively again (since that would be a repetition), and point out some general observations and comparison between the three sections/parts.

• You can also add a summary and outlook subsection to the final conclusion chapter, which will make the last conclusion stand out from the earlier ones, while conveying that this is where it all draws to a close in this work, but is hardly the end as far as the spirit of this investigation goes and that there are future directions which you would ardently pursue.

Of course, my answer assumes that at your university, scholars have the flexibility to organize their thesis the way they want. Please check first that there is no rigid organizational paradigm that everyone is supposed to conform to.

Hope that helps :)

I think there are two parts to your question: 1) how to structure your thesis, and 2) what to call the different sections of your thesis. Your advisor's suggestion for the structure seems quite reasonable to me, and it's how my thesis was structured. As for the names of the sections, I too would prefer not to repeat the names "Introduction" and "Conclusion" for each chapter. But there are ways around it. Here's an example of how I structured one of the middle chapters of my thesis.

                             Chapter 5

The introductory material for the chapter goes here. I didn't give 
this section a name. It's just text that goes before the first topic 
in the chapter.

5.1 Some Topic

...

5.2 Another Topic

...and so on...

5.9 Summary

The "conclusion" for this chapter goes here.

I suspect that what your advisor cares about is the structure, rather than the names of the sections, so you might give him something like this for your next draft. If it turns out he does care about the names, then I'd follow his advice. This might be a convention in your field, and presumably your advisor knows what the external examiners expect in a thesis.

As your thesis begins to take shape, you might find that the best way to present your research is to modify this structure somewhat. You are the expert on your own research, and your advisor is the expert on what is expected for a thesis in your field, so the two of you will probably need to compromise on some things. I had some lively (but friendly) debates with my advisors about the structure of my thesis, and I think the result was all the better for it.