Charlie,
I'm not sure what your question is. Each "row" in the matrix is a "category." To add categories, just edit the list of rows in the matrix.
In your case, you would create a matrix with 3 rows and one column containing a single line text input that restricts values to be integers.
I think I found the survey you were trying to create and it looks like you created header rows rather than "normal" rows. A header row only displays text and does not display inputs. It is used for visually grouping the rows. A "normal" row allows the user to enter data. If you change the rows to normal (by clicking the "Add/Edit Rows" button, then save the matrix you will see the text inputs that you're looking for.
-N
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