Assuming the dice are six-sided and fair:
A fair D6 only has one three among its six numbers. So, 1:6, or about 16.7%.
Ooh, two dice! Now there’s two threes among a total of twelve possibilities…but wait! Two over twelve is the same as one over six! So… 1:6, or about 16.7%.
Hope this helped!
Cheers =D