Hi Michelle
Bnb do this for a good reason. It protects both the host and the guest.
Three weeks isn't a long stay at all and your guest should be able to glean all the information they need from your photos, description and interaction via email on BnB.
It's really not a good idea to suggest someone visits before they book. They will then know your address which present possible security issues.
Many experienced hosts would see someone who is local and wants your contact details as a 'red flag' and decline the booking. If they are local why do they need to stay in your home?
It's probably worth familiarising yourself with the rules of being a host to ensure BnB is the right site for you.
Good luck
Helen