pka's method is certainly the simplest way to do this. Another, applicable if you had a larger number of "tosses" where it would be difficult to right out all possible combinations is this:
The probability of any one coin coming up heads is 1/2. The probability of all three heads is (1/2)(1/2)(1/2)= 1/8.
(The denominator matching pka's 8 possible outcomes of which "HHH" was one.)
Similarly, the probability of any one coin coming up tails is also 1/2 so the probability of "heads, tails, tails" is (1/2)(1/2)(1/2)= 1/8. But there are \(\displaystyle \begin{pmatrix}3 \\ 2 \end{pmatrix}= \frac{3!}{2! 1!}= \frac{6}{2}= 3\) ways to have one head and two tails (those are pka's "HTT", "THT", and "TTH") and each has the same (1/2)(1/2)(1/2)= 1/8 probability: the probability of two tails and one head- in any order is 3/8.
Add: 1/8+ 3/8= 1/2.