Once per row is distribution of three: three values of an attribute each appear exactly once in every row and column, in any order. It is the rule the prep industry calls a Latin square. Its rare cousin, distribution of two, has two values and an empty place; Carpenter found it the hardest rule on the form, and the bank marks only a handful of items with it, so it is taught here rather than drilled alone.
What you see: the same three things reappearing in different places. What you do: find what row three and column three are missing.