The possible outcomes when a coin is tossed once is either a Head $\left( H \right)$ or a Tail $\left( T \right)$.

Then, when $1$ coin is tossed once the sample space $ = 2$

Thus, when the coin is tossed $3$ times the sample space $ = {2^3}$

$ = 8$.

Therefore, the sample space when a coin is tossed three times is $S = \left\{ {HHH,THH,HTH,HHT,TTT,HTT,} \right.$

$\left. {THT,TTH} \right\}$