As phan801 commented, the first interpretation, a sequence of $n$ operations, each of which is either push or pop or multipop, is correct.
Either one of the other two interpretations might stand a small chance without surrounding context or with a very different context. However, had it been the intended meaning, "the phrase would probably have a different structure", such as, "n push operations, n pop operations, and n multipop operations" or "n operations, each being a push operation followed by a pop operation followed by a multipop operation".
A strong indication of the actual meaning comes from the introductory sentence of this section, "17.1 Aggregate analysis".
In aggregate analysis, we show that for all $n$, a sequence of $n$ operations takes worst-case time $T(n)$ in total.
The interpretation 2 means three sequences of $n$ operations instead of "a sequence of $n$ operations".
The interpretation 3 could have been more possible if push-then-pop-then multipop is a reasonable combination. However, after push-then-pop, multipop operation will always be a non-operation, since the stack is assumed empty initially. There is nothing interesting to analyze for this interpretation. (Even if the stack is not empty initially, we would probably use "push-then-multipop" since we can combine a pop operation followed by a multipop operation into another multipop operation".) So this combination of three operations as one operation does not make much sense.