The minimum total you can get when rolling three fair dice is 3, so the probability of getting a sum of 2 is precisely 0 because the event is impossible. The probability of getting a sum greater than 2 is 1 because this is certain.
To be more precise, the probability of two of the dice having a sum of 2 is the probability that one shows a 1 and the other shows a one. This is (1/6)(1/6) = 1/36. However, we also neeed the third dice to show a zero. This probability is 0 because it is impossible. So, getting a 1 and a 1 and a 0 is (1/6)(1/6)(0) = 0
The probability is precisely zero