I think you perhaps overlooked or misunderstood this part:

For example, for sets A = {1, 2, 3} and B = {2, 3, 4}, A △ B = {1, 4}.

Symmetric difference is a binary operation, which means it operates on only two elements. So to evaluate an expression involving symmetric differences among three elements ( A △ B △ C ), you must complete one operation at a time. Thus, for sets A and B above, and C = {2, 3} , A △ B △ C = (A △ B) △ C = {1, 4} △ {2, 3} = {1, 2, 3, 4} .

In other words, the symmetric difference is only defined for 2 sets, so for the examples with more than 2 sets, you take symmetric differences in pairs from left to right, using the last calculated symmetric difference, as needed.