Using two normal dice, what is the probability of rolling numbers that add up to 3?

That would be rolling a 1 and a 2.

so possibilities are 1 on first die and 2 on the second die

OR 2 on first die and 1 on the second.

Both of these have a probability of 1/6 * 1/6 = 1/36

so required probability = 1/36 + 1/36 = 2/36 = 1/18