Just send an email to their public diplomacy service or press services if they have one, and ask them if it’s possible to get all three books delivered. They’ll be happy to know you’re interested.
Your embassy will be located in the capital of your country, you have to contact the Chinese embassy obviously, and they speak your local language.
Is there a specific way one should ask? It’s seems a bit awkward to do it out-of-the-blue.
Personally I just said that I heard they had this offer (didn’t mention prices though) and that I’d like to get volumes 2 and 3 if possible, and they replied the next day saying thanks for your interest in China and the writings of President Xi, asked my adress, and said they would send them my way.
If they have the offer they already expect people to get it, so don’t worry about asking haha