I often wonder if the threefold law is commonly misinterpreted. Rather than interpreting it as "what you put out will return magnified by three," I wonder if it's a more subtle, symbolic statement. For example, perhaps it refers to the fact that everything you do (and this goes for mundane as well as magical actions) will affect you on all three levels of mind, body and spirit. People often think of magic as acting to physically change the world around them, but the reality is that magic has just as big of an effect on the magician him/herself as on the outside world. Arguably even more so -- the understanding of magic upon which I operate is that it works first and foremost by transforming the magician and secondarily by manifesting change in the physical world. When you perform magic, you initiate a transformation within yourself on a deep spiritual level, which trickles down to impact your mind, whose workings are closely tied to the biological churnings within your body. To me, the threefold law is a reminder of that chain reaction.
As for "an it harm none," I think that has a lot to do with how you define the word "harm." Again, I perhaps have a different understanding than many witches. What does "harm" really mean? It's a huge gray area. Personally, I've found that practicing magic has led me inward to a deeper understanding of my (eternal) self and what's truly important. All this stuff we do in the physical world -- it's life-and-death to the ego, but once you step back from the ego it begins to look more like a great big game, frolic, stage play or art installation created by the universe. All these things the ego considers "harmful" can't ever touch the little shard of the divine within each of us.
So once you get past the ego, what CAN truly harm us? The answer, I think, is determined by what you perceive your purpose in life to be. For my part, I think we are simply here to make choices and to experience life through our own unique, authentic lens -- in a sense, we are living works of art. Therefore, in my mind, the greatest harm you can do to another person is to interfere with their free will. By that measure, doing a spell to help you get a job in no way impinges on another person's freedom to pursue the same job. However, I do feel doing a spell to directly force change upon an unconsenting individual is a gross attack on free will.
This, of course, is my personal spiritual approach to magic, which may or may not resonate with someone who is more ceremonially inclined. I read "When, Why... If" years ago as a beginning witch and don't really remember its specific contents, but I do feel it was valuable in that it got me thinking seriously about the potential consequences of magic. Great topic!