If there are two threes then there are 4 remaining numbers on a cubic die. So the probability is 2/6=1/3 of casting a 3.
If there are 2 3s, one other number, 1, 2, 4, 5, 6 must be missing. If this number happens to be 1, then the probability of 1 is zero. The probability of 1 or 3 is 1/3.
If the missing number is not 1, then the probability of casting 1 or 3 is 3/6=1/2, because three faces contain 1, 3 and 3, and three faces contain other numbers.