Based on the creative story provided, I'll think through this logically. The question "Why can't a three-legged cat play the piano?" appears to be a setup for what could be either a riddle or a logical exploration.
 
Let's analyze this step by step:
 
1. A typical cat has four legs
2. A piano requires paw/hand movements across the keys
3. The story introduces Triskel, a three-legged cat
 
The answer likely isn't that a three-legged cat is physically incapable of playing the piano, since many pianists can play with physical differences or limitations. Even with three legs, a cat could theoretically reach the keys.
 
Given the whimsical nature of the story and the setup, I suspect this might be leading to a punchline or revelation. Is there perhaps a wordplay element I'm missing, or would you like me to expand on the actual physical and cognitive considerations of a three-legged cat attempting to play the piano?