A suitable sample space would be the results of the three missile launches: intercept, miss, or hit. $\\{{\sf I, M, H}\\}^3$ is an acceptable representation for this. However, you hardly need to use this.
The results for a particular missile launch are _mutually exclusive and exhaustive_ events, since a launched missile may either be exactly one of intercepted, miss, or hit; not anything else, and not any combination.
The three launch results are pairwise independent events, since the results for any two particular missiles are presented as having no influence on each other.
Your favoured event is $\\{({\sf H,H,H})\\}$ and indeed is $\mathbb P\\{({\sf H,H,H})\\}= {(\tfrac 23\tfrac 34)}^3$, which comes from the probabilities given and the pairwise independence between the results for different missiles. (Also the definition of conditional probability).