So you have "LetterSprite" with instance variable "letter" (which stores Y, E, S).
To create random letters you have something like this:
Repeat 10 times
LetterSprite set letter=choose("Y","E","S")
LetterSprite set animation to self.letter
And you have, say "ResultSprite" that somehow recovers falling letters.
Create instance variable ResultSprite.word
Each time ResultSprite recovers a letter, set:
ResultSprite.word=ResultSprite.word & LetterSprite.letter
Once 3 letters are recovered, check if:
I hope I understood you correctly, if not could you share your capx?