A multiple of 3 which is also a multiple of 7 is a multiple of 3×7=21. The minimum score after rolling 3 dice is 3 and the maximum is 18, so a score of 21 is not possible and therefore the probability of rolling a multiple of 3 which is also a multiple of 7 is zero.
There are 63=216 possible outcomes of rolling 3 dice.
The probability of rolling a multiple of 3 is 72/216=⅓; the probability of rolling a multiple of 7 (7 or 14) is 30/216=5/36. These are independent probabilities, because both cannot occur together.
The probability of rolling either a multiple of 3 or a multiple of 7 = ⅓+5/36=17/36.