@Hilda78 Emiel's response seems to assume you meant that the guest is requesting 2 nights when your minimum is 3, and that you want to be able to accept it, but I read your post as meaning that you are asking how the guest can ask to book for 2 nights when your min. is 3. If that is correct, then guests can actually ask for anything their little hearts desire- that you let them come with their 80 pound constantly shedding dog when your listing states "No Pets", that they can book for themselves and their 4 children, when your listing says "No Children", that you prepare them a gluten and lactose-free breakfast, when breakfast isn't offered at all, that you wait up until 2 AM, because that's when they want to check-in.
When guests make requests that you cannot or do not want to accomodate, and which contravene your Rules or settings, just message them back saying "Hi XX, thanks for your request, I'm sorry, but as you can see from our booking settings, we only host 3 night minimum stays. If you wish to book for 3 nights, I'll be happy to accept, otherwise I suggest you look for another listing that meets your needs."