Yes, it's legal. WRONG. Once ideas are recorded, they are called intellectual property. This property is owned by the creator--even if there is no copyright symbol.
No, it's mot legal. Whether you used someone else's intellectual property knowingly or accidentally, it is still plagiarism. Any act of deceiving or misrepresenting is illegal.
Yes, it's ethical. WRONG. You must cite the name of the creator, the name of the work, where you found the information, and the date of your Internet visit. You must also preserve the intent of the creator's words.