The threeLetterCode type defines a string of 3 characters, only (capital or small) letters.
Example:
<currencyCode>EUR</currencyCode>
XML Schema:
<s:simpleType name="threeLetterCode">
<s:restriction base="s:string">
<s:pattern value="^[A-Za-z]{3}$"/>
</s:restriction>
</s:simpleType>