The sample space is the set of all possible outcomes of the experiment. So for tossing three coins, and denoting each coin's result as H or T, the possible outcomes are:
{HHH,HHT,HTH,HTT,THH,THT,TTH,TTT}
So, we can see that one outcome (HHH) corresponds to getting 3 heads at a time, and one outcome (TTT) corresponds to getting 3 tails (hence the probability of each is 1/8, i.e. one outcome of 8 possible outcomes total).