I think you always get 3 attacks of 9 strength, vs a fixed defense since each unit is attacked individually.
IMHO it should have used another way of calculating these values.
For units it should have been a different notation like 3x9/7/2 (3 figures, 9 attack, 7 defense, 2 moves)
but finding a neat way to represent stacks of different units with different values won't make any sense anyway.
An alternative would be to calculate for the entire unit a single tactical strength number factoring everything together, that is simply summed to the other units in the stack to come up with a stack strength value.