TERRARIUM - Anthophobia Dev blog

Oh, the 3rd one! I'm really sorry, my mistake.
You can unlock it by opening the locked door at chapter 5 (Kindergarten area place where you can find gold cards)
Do you remember where you found the card key that unlock the door to boss fight? it's the door right behind that card.
Also in order to open that locked door, you need to find 4 gold cards in that area.