Exp. (a)
There seems to be ambiguity in this question. It should be mentioned that boxes are different and one particular box has 3 balls.
Then, number of ways =31212C3​×29​=355​(32​)11
According to the question,
3123C1​×12C3​29−3C2​12C3​9C3​+3!3!6!3!12!×3!​​