Q. What is the distinction between yeah, yea, and yay? Is each confined to a specific usage?
A. Dictionaries are terrific for looking up what words mean. I found all these words at Merriam-Webster’s free online dictionary.
Yay means “hooray”; rhymes with day
Yea means “yes” or “indeed”; familiar to many from translations of the Bible; often used in voting (“yea or nay”); rhymes with day
Yeah means “yes”; famously used by the Beatles (“She loves you, yeah, yeah, yeah”); rhymes with pretty much not anything (bleah?).