Each flip is independent of each other.
For single flip, the probability of getting a head would be 1/2 because there are two outcomes in total (head and tail), and there are one desired outcome (head).
Now for three flips, we need 3 heads. So we need head for first flip, second, and third too, so that would be (1/2)(1/2)(1/2) = 1/8
Final answer: 1/8