Probability is a ratio of the number of relevant outcomes and total number of possible outcomes.
We are flipping the coin three times.
The number of relevant outcomes is 1 (one triple {Head, Head, Tail})
The number of all possible outcomes is the product of 2 possibilities (Head or Tail) on first toss, times another 2 on second, times another 2 on third, so 2^3 = 8.
