First, tabulate all three measures. There are two ways we can compute a scale. One method is to add up all the indicators; the other is to take their average. In this case, we will take the average. This is because we can take an average score even when someone missed a question$$we cannot add them up unless respondents answered all three items.

Select transform$->$compute.

Enter the following into the $$numeric expression$$ field and then press OK: MEAN$\mathrm{.}2($V$7329,$ V$7330\_$r$,$ V$7333).$ This syntax can be read as $$take the average of these three variables if at least two of the three items have been completed.$$