You need to account for the fact that, if you get all three prizes, one of them comes twice. So you could get $AABC$, $CBCA$, etc. The number of ways to choose a permutation of two $A$'s, one $B$, and a $C$ is $\binom{4}{2} \times 2 \times 1 = 12$ (the number of ways to count the other similar permutations is the same). The probability of any such permutation occurring is $(0.5)^2(0.4)(0.1) = 0.01$. Similarly, the probability of a permutation where $B$ is repeated is $(0.5)(0.4)^2(0.1) = 0.008$ and the probability of a permutation where $C$ is repeated is $(0.5)(0.4)(0.1)^2 = 0.002$. Adding over all such permutations gives a $0.24$ probability of getting all three prizes.