You will need Enterprise license for a cluster size above 8, and a 3-site cluster may exceed 8 nodes. XDR synchronized clusters are also supported only in the Enterprise version. If you are running with an Enterprise version, I would suggest you reach out to Support to have consultation with a knowledgeable solution architect. If not and if you are ok to be contacted by email, I can provide your email to someone who can discuss the options. (Corrected the Community Edition cluster size limit to 8.)