It's kind of weird, but I think the best way to get back at her is to remain NC. She has no idea what you're thinking, what your reaction is. That up and down nagging of 'Does he hate me? Is he sad? Is he just moving on like I didn't even matter either?". That last one is probably the most powerful, and can only be done by NC, and perhaps indirect information provided to her by friends, hearsay... whatever - that you're simply moving on with life. With NC, you're letting her own mind work against herself. If she doesn't know your emotional state in regards to her... then it can play on in her head for a long time. Much longer than if you give her an emotion for her to settle on (such as anger, sadness, rude, snottiness, rub in her face... etc.).
Downside is that by virtue of it being caused by NC, you don't get to see or know that it's affecting her either. That's kind of the tricky part... which I have to admit I have trouble with as well. You want that feeling of 'Yeah, I showed her!', or you want to see her in regret for causing you pain. You want to see the fruit of your actions. This is where you have to rationalize for yourself... that this will produce the biggest fruit. And she'll make it by her own hand, rather than you trying to drive a dagger in for the last go.
Sometimes it would be great to just 'know' it was working, and it was best... that's where I have to have faith in my past experiences, as well as advice from other people here.
You guys were together for three years. She won't forget that. She simply can't, even if she was completely sociopathic or whatever. She'll be looking at how you handle the aftermath of this. If you ever did want to communicate, I'd advise to come from a place of indifference and 'all the best to you'. Where you're content to let things go and you're just moving on. Basically - where you would be at the end of all this. It's a bit more of a burn to know someone is just pressing forward and moving on, rather than if they're still held back, holding emotions and anger towards you.
That's my take on it anyway. One thing other members in the past have recommended is writing a letter as intended for her...and then never sending it. Definitely a terrible idea to actually send one (others have sent one and have paid in silence in return or say a patronizing short reply 3 weeks later).
Also, try and not get into the habit of thinking about her too much (I made this mistake, paying for it a bit now). It will ingrain in you as a habit, so you get used to thinking about her constantly. So try to think about other things (or nothing at all).
