From 24 total letters available, we're picking 3 (without replacement). The number of ways of doing so is
[tex]P(24,3)=\dfrac{24!}{(24-3)!}=\dfrac{24!}{21!}=12,144[/tex]
In other words, we're counting the permutations of 24 objects taken 3 at a time. (Permutations because, presumably, Alpha Beta Gamma is not the same as Gamma Alpha Beta, for instance.)