We know that, in a Bindmial distribution,
(i) There are 2 outcomes for each trial
(ii) There is a fixed number of trials
(iii) The probability of success must be the same for all the trials.
When coin is tossed, possible outcomes are Head and Tail.
Since coin is tossed three times, we have fixed number of trials.
Also probability of Head and Tail in each trial is 1/2.
Thus given experiment is said to have binomial distribution.