# Elementary combinatorial question

I've tried to solve the following exercise (Ross First Course 8th edition):

I have to chose three students from three groups of $n$ students (in total exist $3n$ students):

If I must choose two students of the same group and the other from a different class? My try: First I chose a class of the three, then two students of that class, a class from two other class and finally an student: $$3 \binom{n}{2} 2 \binom{n}{1}$$.

Then the next exercise is: If emphasized textthe three students must be in different classes My try: $3 \binom{n}{1} 2 \binom{n}{1} 1 \binom{n}{1}$. But the solution in the textbook is $n^3 = {\binom{n}{1}}^3$.

What's the difference?