Group the given figures into three classes using each figure only once.

In a figure-classification question, the goal is to find one structural attribute that is common to the figures within a class but differs across classes -- for example, the number of straight strokes used to draw each figure, whether the figure is open or closed, or how many enclosed regions it has. Once that attribute is fixed, every figure sorts into exactly one class.

Here, the distinguishing attribute is the number of straight-line strokes needed to draw each figure. Counting the strokes in every figure gives:

Each class shares exactly one stroke-count, and all nine figures -- 1 through 9 -- are used exactly once across the three classes, so the classification 1, 2, 3 ; 4, 5, 6 ; 7, 8, 9 is the valid three-way grouping.