You just have to calculate the probabilities of both cases: rolling 8 with 2 dice, and rolling 8 with 3 dice.

Starting with the 2 dice: which combinations of dice rolls yield 8?

- 2 with dice1, and 6 with dice2

- 6 with dice1, and 2 with dice1

(these are two different events!)

...

Count them, and count how many different combinations of dice rolls (with two dice) are possible overall.

Before you start with the 3 dice, it helps to look at the probability distribution of 2-dice-rolls, as there's a simple pattern to it.