XOR cipher with three different ciphertexts and repeated key, key length known. How do I find the plaintexts?
You're using bytewise xor, and three plaintexts $x,y,z$ with a small alphabet (A-Z, which I assume means ASCII values 0x41-0x5a). Then knowing $a \oplus x$, at a certain index, we know all possible values of $a$, at that same index, namely $\{0x41 \oplus (a \oplus x),\ldots, 0x5a \oplus (a \oplus x)\}$. But we have 2 more constraints from $a \oplus y$ and $a \oplus z$ at that index. The intersection of these three sets is the set of options for $a$ at that index. This already reduces the options at each index, probably.

Then you have to use probable plaintext, say 'THE' (if you have English text). You can try 'THE' at all positions as part of text $x$, compute $a$ on these corresponding places, and then compute $y$ and $z$ as well. Check whether the results look like (parts of) English words, and try to extend words in those other texts. Also try 'THE' in texts $y$ and $z$ in all positions. You can immediately abort all attempts where $a$ is not in the sets we computed above. Google "reading in depth". With three texts this should give pretty fast results. All of this is easily automated.

Doing statistics only on 21 characters will not get all the way, I think. You do need some idea of the language for the plaintext, etc.