It doesn't matter! and the other 5 permutations of the vectors differ only in sign. Since the sign does not matter.
In fact, with , , and , since , it is easy to see that .
Permuting the vectors just permutes the rows which only changes the sign. And, again, the volume is the absolute value of that.
(Three points determine a plane. Any two vectors, having the same "initial point", lie in a plane.)