The probability of getting a correct answer on a given question is 1/2
To get all 3 questions correctly, then you must guess the 1st question correctly, as well as the 2nd and 3rd questions
P(all 3 correct) = P(1st question correct)*P(2nd question correct)*P(3rd question correct)
= (1/2)^3 = 1/8
hope this helped