For "at least" questions, it is useful to calculate the probability of that event not happening, In this case, the probability of not getting one three (or any specific number). It is quite obvious that for one dice you have a 5/6 chance of not getting a three. For three the probability of not getting a three would then be:
5/6⋅5/6⋅5/6=125/216=0.5787
So the opposite (getting at least one three) would be 1 minus that or 1 - 0.5787 = 0.4213
