Final answer:
The third syllable intoned by the giant from 'Jack and the Beanstalk' is 'fee' as part of the chant 'Fee-fi-fo-fum'.
Step-by-step explanation:
The third syllable intoned by the giant who smells the blood of an Englishman is "fee." This is from the traditional English fairytale "Jack and the Beanstalk" where the giant says the famous lines, "Fee-fi-fo-fum, I smell the blood of an Englishman." The giant's chant is often used to indicate the presence of a stranger or an intruding human in the giant's domain.