First off, let's get one thing straight. Wiccan Ethics are more than just the motto "Harm None", which does not mean "Don't hurt anything, ever, because if you do, it will come back to bite you three-fold". Wiccan ethics are actually a lot simpler than that, and yet a lot more complex. That's the paradox of ethics.
Let's start with examining the Harm None couplet: "An ye harm none, do what ye will". Translated into modern English, it means, If it does not cause harm, do what you like. Anything that doesn't cause harm is fine to do. That's an enormously freeing statement.
It means that we aren't bound by someone else's moral code, only our own. All acts of pleasure are sacred to us. But what about actions that do cause harm? Isn't that where the Law of Three comes in? Yes, and no. The Law of Three really isn't a law, per se, and it's hard to find any sources for the origins of this principle in early Wiccan writings.
There are some sources for a Law of Return, which basically states that whatever you put out you'll get back. Or, in other words: you reap what you sow, good and bad. This makes sense, given the physical law of equal and opposite reaction: whatever you put out into the world will also push back on you. Of course, this also depends on how one defines harm.
How do we know which actions will cause harm and which will not? Harm can be defined as deliberately hurting someone or something unnecessarily. The two keywords there are "deliberately" and "unnecessarily". Deliberately implies conscious intent. You intend to cause harm with your actions, therefore you are aware of what you are doing and ought to be prepared for the consequences.
Unnecessarily implies that there are actions that will not cause harm that you choose not to do. Looking at this definition, you will see that it leaves out key things like killing something to eat it, and self-defense. These are necessary harms. You must kill a plant or animal in order to get the nutrition you need to survive. Sometimes you must hurt or kill something that's threatening you in order to survive.
It also leaves out unintentional things, like stepping on a bug or accidentally running over a squirrel with your car. These actions come without consequence. No discussion of Wiccan ethics would be complete without examining two other key points: personal responsibility and free will.
You'd be hard-pressed to find a Wiccan that doesn't believe you must take responsibility for your actions. Even those who believe the three-fold thing literally, they believe that you'll be forced to deal with consequences. If you take it upon yourself to do something, then you must face the consequences.
Speaking of magical action, this leads us into the principle of free will. Many believe it is unethical to use magic against something's free will. Simple idea, but this concept brings its own round of tap dancing. Some claim that love spells don't violate another's free will because it's good for them.
Some claim that bindings don't violate another's free will because it's good for you or perhaps good for them as well. That's all rubbish, really. If we are going to take magical action, we are judging what we think the best outcome should be.
Does it violate anyone's free will? What about bindings? While it might seem stupid to work a binding spell to keep a co-worker from gossiping, what about doing one to keep an illness from spreading in a person? By binding or banishing a cancer, are you interfering with the free will of the illness?
Ethics are a very personal thing. The teachings of Wicca give you guidelines for how to act, magically and mundanely, but that's all they really are. In the end, it's up to you to decide how to apply those guidelines to your life, how you will define harm, how you will define free will, and what consequences you are willing to accept.