When your ex suddenly texts you after you've tried to get into no contact for a bit, it can be really hard to resist temptation. Fortunately there are ways to stay on target and hit your goal of 30 days NC, or however long you've set. 

Basically, you need to: 

1. Resist the temptation to text him back.

2. Continue on your plan to stay emotionally healthy after the breakup. 

Now, I'm assuming you do want to get back together with your ex at a later point, but only after you've had a chance to go no contact for at LEAST 30 days, because that lets you avoid the "panic mod" that he'll have expected you to fall into. 

It also lets your reassess your goals fro the relationship and what you want out of your time with him. It allows you to take back control in the relationship. 

In order to resist the temptation of texting back your ex, you can either delete his text, or simply put it out of your mind. Save it for later, basically. 

In a week, you can respond and say, "Hey! I just got your message. I was..." and then make up or tell him whatever you've been up to - as long as it's a positive activity. 

Remember- you want all your future interactions with him to be POSITIVE and focused not on the breakup, but on the new kind of relationship you want to establish. 

What kind of relationship is that? One based on mutual respect, love, and trust. You are going to be the one who sets the pattern now. And that pattern will be open communication and honesty as well as a "no fighting" type of relationship. No matter how heated things get between you, you're always going to be the one to back off and say, "I'm not talking about this until we can do so calmly." 

That's how you set the new pattern for a healthy and positive relationship with your ex. 

So remember: when he texts you after 3 weeks of no contact, your job is to resist the temptation to write him back. Instead, stick to your guns and keep NC for the remainder of the month. 

Then, when the NC period is up, you can contact him and say you just got his text. You can then arrange to meet somewhere informal - try a coffee shop or something where it doesn't feel especially like a "date" or anything too pressuring. 

You want it to feel like a casual get-together as friends rather than a lovers' rendezvous.