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.
Do you think I could call them instead? I’m not very good with emails, and I have no idea how to write a formal email.
Probably, but you don’t even need to be very formal. Just say you heard about the offer and that you would like to get all three books, thank them for their time, sign your email and send it.