1 When we list three items, we always have to ask ourselves whether the third point is something special or extra, something that we want to give particular emphasis. See example 1.
2 Of course, we could also emphasize this third point by adding a comment. See example 2.

Please note: in American English this comma, aka Oxford comma, is used as a standard comma. However, I like to make use of this subtle difference of deciding whether to set the second comma.

1 At McVeggie’s Restaurant you can always get a job, earn some money, and meet all the IT people of the town.
2 At McVeggie’s Restaurant you can always get a job, earn some money, and, what is phantastic, meet all the IT-people of the town.

Let’s have a look at my essay again.

1 As the third item is, in my view, more important than the first two, and somewhat longer, I am inclined to put a comma there.
2 Of course, I can also emphasize it with additional information in a comma bracket.

1 There is a number of studies that challenge this concept of inferiority and demonstrate that animals can feel, learn, and look forward to pleasant consequences of their action.
2 There is a number of studies that challenge this concept of inferiority and demonstrate that animals can feel, learn, and, most importantly, look forward to pleasant consequences of their action.

There is one superordinate rule in English punctuation: that is readibility. Where adjoining verb phrases can cause misunderstandings or lead to misreading, there should be a comma. This is a general rule and can outweigh other rules. In our example, ham and eggs belong together, so a second comma becomes necessary to prevent misreading.

His favourite dishes are chicken curry, roast beef, and ham and eggs.