When you divide your text into subsections you will have to make your reader understand what you are doing and what you are doing it for. This means that all sections at first and second level that you split up into subsections get an introduction.
Third level sections usually don’t get an introduction as they are not split up. You would get a fourth level, which is really too much of a good thing. The example below shows a first level section that is subdivided into two second level sections in which two systems are compared. Notice that Chapter 3.1 is not split up into fragments and therefore does not have an introduction.
Please bear in mind that in reality you would never call them system A and B. Give them names that explains how they are different form each other.
Chapter 3: Comparison: System B performs better than system A
Now that we have established that system A and system B both yield satisfactory results, it is time to see which system meets the requirements best. Chapter 3.1 shows how both systems compare on durability and maintainability, chapter 3.2 will show how they cope in extreme situations.
3.1: Both systems are durable and easily maintainable
According to Johnson (2015), System A was found to be extremely durable. Even after 15 years of non-stop service the accuracy remained …. This was confirmed in several experiments (e.g. Post 2011, Williams 2012). System B, likewise, has proved to be ….
Maintenance of both systems was different only in minor aspects. System A requires slightly more time for …., but his is negligible because …
3.2: System B is more reliable under extreme conditions
Even though both systems function similarly under most circumstances, some remarkable differences came to light when we tested them in extreme temperatures. Both systems were used to measure X at 12 different temperatures and it was here where system B proved to be … Chapter 3.2.1 will first show the results of …., chapter 3.2.2 describes …
3.2.1: Low temperatures
Our measurements show that at temperatures below 100K, system A started to show …. This means that an extra insulation …., which will lead to ….
3.2.2: High temperatures
The last two measurements, at 500 and 600K respectively, show a marked difference ….
3.3: Conclusion: system B will be more reliable in future deployments
In the current situation, system A and B will both work equally well, as the temperatures at which they are used are moderate. However, as the situations, and with these the temperatures at which the systems will be subjected, will become more extreme in the next 3 or 4 years ….
(Do you see, by the way, how all the headings together form a kind of summary?)