I'm specifically talking about the first three. One was from Barty Crouch Jr. (disguised as Moody) in Harry Potter and the Goblet of Fire Chapter 15:
Harry moved forward into the middle of the classroom, into the space that Moody had cleared of desks. Moody raised his wand, pointed it at Harry, and said, “Imperio!” It was the most wonderful feeling. Harry felt a floating sensation as every thought and worry in his head was wiped gently away, leaving nothing but a vague, untraceable happiness. He stood there feeling immensely relaxed, only dimly aware of everyone watching him.
And another one was from You-Know-who after Harry had reached the Graveyard in Chapter 34:
“I asked you whether you want me to do that again,” said Volde- mort softly. “Answer me! Imperio!” And Harry felt, for the third time in his life, the sensation that his mind had been wiped of all thought. . . . Ah, it was bliss, not to think, it was as though he were floating, dreaming . . . just answer no . . . say no . . . just answer no. . . .
The book specifically mentions that this was the third time Harry was feeling the effects of Imperius curse. I've tried searching for the second time he was cursed, but couldn't find it anywhere in this book.
Was Harry cursed before this book? Or did I miss it somewhere?