The total number of ways 5 people can be selected out of 8 is 8C5 (combination of 8 taken 5) which is equal to 56. Given the conditions that at least three girls should watch the movie.
(1) girls - 3 ; boys - 2 ; (5C3) x (3C2) = 30
(2) girls - 4 ; boy - 1 ; (5C4) x (3C1) = 15
(3) girls - 5 ; boy - 0 ; (5C5) x (3C0) = 1
Adding all the three possibilities, 46. The probability is the ratio of this value and the total number, 56. Thus, the answer is approximately 0.80.