How many comparisons in the worst case, does it take to merge 3 sorted lists of size n/3?
First of all, we have to distinguish between two different questions. The first is about analyzing the specific (generalized) merging algorithm used in merge sort. The second is about analyzing the problem.

Merge sort merging

The paragraph you quote attempts to analyze the merge sort merging algorithm. It only gives a lower bound on the number of comparisons, though presumably it is easy to show that it is also an upper bound. In order to give a lower bound of $C$ on the number of comparisons, it is enough to describe a situation in which the algorithm uses $C$ comparisons. Such a situation is the "worst case [instance]" that the paragraph is talking about.

The merging algorithm starts with three lists of length $n/3$, and constructs a "big list" of length $3$. The algorithm works by comparing the smallest elements in all remaining lists, and moving the smallest one to the top of the big list. As the algorithm progresses, eventually lists will empty &mdash; all their elements have been moved to the big list.

The paragraph suggests to consider the situation in which the first list empties when the other two have exactly one element remaining, which it claims is a "bad case", indeed the "worst case"; for the sake of the lower bound, we don't care whether this is actually true. You have to argue that the situation can actually happen, and carefully count all comparisons done by the merging algorithm. The merging algorithm the paragraph considers always compares the bottom elements of all remaining lists, requiring $\ell-1$ comparisons if $\ell$ non-empty lists remain. In this case, as long as all lists are non-empty, we need two comparisons per element.

At the situation considered there are only two elements left, so $n-2$ elements have been processed with two comparisons per element, leading to $2(n-2)$ comparisons. The final two elements require one more comparison. It is not too hard to check that this is indeed the worst case: this situation maximized the number of times that an element requires two comparisons to process.

The more general case with $c$ lists is very similar. Again the worst case is when the first list empties when all other lists have exactly one element. Up to now there have been $(c-1)(n-c+1)$ comparisons. The next element will require $c-2$ comparisons only (since there are only $c-1$ non-empty lists), the element after that $c-3$, and so on. This gives the formula you state.

General question

A different question one can ask is: given $c$ sorted lists of length $n/c$, how many comparisons are needed to determine the sorted order of all of them merged. There are two variants of this question:


What is the minimal $C$ such that there is an algorithm which outputs the correct order and never uses more than $C$ comparisons; the algorithm is only allowed to access the lists by comparing two elements. This is the so-called decision-tree model.
What is the minimal running time of an algorithm merging the lists.


These questions are rather different. The first question is much easier to answer, and I will only comment on it. However, the minimal $C$ doesn't necessarily correspond to an efficient algorithm, since determining which comparison to do next could be difficult.

The classical lower bound on sorting extends to this situation. Given $c$ sorted lists of length $n/c$, the number of possible ways they fit together to one big list is $\frac{n!}{(n/c)!^c} = \Theta\big(\frac{c^n}{n^{(c-1)/2}}\big)$, and we obtain a lower bound of $(\log_2 c)n - O_c(\log n)$. Fredman's classical result shows that there is a corresponding algorithm in the decision-tree model using at most $(2+\log_2 c)n$ comparisons (for large enough $n$, depending on $c$), which is better than the merge sort merging procedure for $c \geq 5$. The correct answer is probably $(\log_2 c)n \pm o_c(n)$. Indeed, using heaps you can merge $c$ lists with $O((\log c)n)$ comparisons efficiently, as described for example in answers to the following question.