The key word here is "may" meaning that we could easily have 3 rational roots as well. An example of a cubic having 3 irrational roots would be
(x-1)(x-2)(x-3) = x³ - 6x² + 11x - 6
This has the rational roots x = 1, x = 2, x = 3.
However, we could easily replace 1,2,3 with any irrational numbers we want. So that's why the statement "a cubic has three irrational roots" is sometimes true.
In some cases, a cubic may only have 1 real root and the other 2 roots are imaginary.
