I absolutely hate these three words whenever I read them in an instructional context. That means any textbook, explanation or paper on a technique. These words just smell of condescension.

• Simple - For example. When someone says "We use a simple technique to do xyz". If it was simple, why do you need to write about it? Maybe it is simple to you doesn't mean it is simple to everyone.
• Obvious - "The solution is obvious and therefore is left to the reader". You remember these in high school text books don't you? If it was obvious, I wouldn't be reading your textbook.
• Clearly - "Clearly because he did xyz, abc occured". No! If it was clear, I wouldn't need to ask your opinion on it. I guess only in the most simple and obvious scenarios can you use clearly. "Clearly it is sunny and warm today".