“He has let the cat out of the bag and it is really an important thing to be noted.”
What is the meaning of the idiom ‘let the cat out of the bag’?

“let the cat out of the bag” itself shows that a cat when stored in a bag, can’t be let out as it may spoil the situation. So when a secret is revealed, we can use the idiom “let the cat out of the bag”. So option 3 is the right one.