Using a binomial distribution considering there's a 30% chance it will rain on any of the three days:
The probability of it raining on 0 days is (1)(0.7)(0.7)(0.7) = 34.3%.
The probability of it raining on 1 day is (3)(0.3)(0.7)(0.7) = 44.1%.
The probability of it raining on 2 days is (3)(0.3)(0.3)(0.7) = 18.9%.
The probability of it raining on 3 days is (1)(0.3)(0.3)(0.3) = 2.7%.
There's a 65.7% chance that it will rain at least once over the three-day period.