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
Complimentary just means that they add to be 90, so you take 90 degrees and subtract the angle you already have in order to get the complementary angle
