I know that it's a very common practice to observe the Threefold Law/Rule of Three, and many people live by it, going out of their way to do no harm for fear of the rule of three and backlash from karma. Personally, I think the idea is all well and good for a lot of people. Intentionally doing harm when it isn't necessary is just wrong, in my opinion. Self-defense is totally different. If someone attacks you and you kick them where it hurts, stab them, whatever you have to do to stay safe, that is what you should do (again, my opinion), and I seriously doubt karma will come up and bite you in the ass later on for it. If you just walk up to someone and punch them for no reason, THAT might bite you in the ass later. I believe the same rules apply for magick. If you do something to protect yourself and those you love, it's self-defense. Striking out at someone for no reason, not cool.
There's someone on YouTube who used to go by HollyDaisyRose, but I'm drawing a blank on her current channel name. She has a blog, and I'm subscribed, so you can find her that way if you want to check her blog or channel out. Anyway, the reason that's relevant is because we seem to share a similar perspective about the Rule of Three. I believe that it's a nice thing and all that, but it's sort of a scare tactic as well, like hell in Christianity. "If you do bad, you're going to be punished severely!" and that sort of thing. No, I don't believe you should hurt people intentionally, but I don't think that you should just blindly follow something, either. If you believe in it, then by all means, observe it and follow it.
Some people say that it doesn't apply if it's self defense. That's personal opinion. Believe what you will about it.
Now, there's the theory that the Law of Three means that whatever you do will come back to you three times. Some believe that means it will come back to you three times as bad, some believe it means that it will come back to you in three ways (such as emotionally, physically, etc.). Then there are some who basically totally dismiss it and just say that it's a metaphor for karma. I believe in karma, all the way. I think that what you do will come back to you, but I don't think it will come back to you in the same way. For example, if you push someone down a flight of stairs, I don't think that you'll be pushed down stairs.
Believe what you like about karma and the Rule of Three, but my advice is to never do anything (magickally or otherwise) that you are not prepared to face the consequences for.