Probably the easiest way to handle questions with the wording "at least one ..." is to find the probability of the opposite situation and subtract from one. The opposite of this is that it rains all three days. If that doesn't happen, then that means that there is at least one rainy day in the three days.
So the probability that it rains all 3 days is (0.4)3 = 0.064. Then the probability that this doesn't happen, i.e., that at least one of the days is rainy, is 1 - 0.064 = 0.936, or 93.6%.
