Usefulness of the bigger mounts

[Alneyan]

Shouldn't it be: 0.125% for 3 hits with the Heavy mounted weapons? It should be equal to the probability of having no hit. If I am right, and if my calculation is correct, it should be 1,5 hits on average for 450 damage on average. (Which would make sense with a 50% chance to hit after all) But I may be wrong given my poor maths skills.