Your question is slightly vague, so I will pose a more defined
question: What is the probability of 3 coin tosses resulting in
heads exactly twice?
This is a pretty easy question to answer. The three possible
(winning) outcomes are:
1. Heads, Heads, Tails.
2. Heads, Tails, Heads.
3. Tails, Heads, Heads.
If we look at the possible combination of other (losing)
outcomes, we can easily determine the probability:
4. Heads, Heads, Heads.
5. Tails, Tails, Heads.
6. Tails, Heads, Tails.
7. Heads, Tails, Tails.
8. Tails, Tails, Tails.
This means that to throw heads twice in 3 flips, we have a 3 in
8 chance. This is because there are 3 winning possibilities out of
a total of 8 winning and losing possibilities.