Final answer:
The word that contains 26 letters but only three syllables is 'alphabetical'.
Step-by-step explanation:
The word that contains 26 letters but only three syllables is alphabetical.
An example of a step-by-step breakdown of the word 'alphabetical' into syllables is as follows:
Al - 2 syllables
pha - 2 syllables
bet - 1 syllable
i - 1 syllable
cal - 1 syllable
So in total, 'alphabetical' has 3 syllables.