Let $A$, $B$ and $C$ be three events such that the probability that exactly one of $A$ and $B$ occurs is $(1 - k)$, the probability that exactly one of $B$ and $C$ occurs is $(1 - 2k)$, the probability that exactly one of $C$ and $A$ occurs is $(1 - k)$ and the probability of all $A$, $B$ and $C$ occurring simultaneously is $k^2$, where $0 < k < 1$. Then the probability that at least one of $A$, $B$ and $C$ occurs is: