Thanks for the answer.
As far as I understand, in a multisite, data is replicated between sites. I need 3 clusters without replication between the clusters, but with the ability to search on all the clusters with one search head.
It is typical for multi-site clusters to replicate data between sites, but you don't have to do so.
site_replication_factor = origin:2, total:2 will keep all copies in the originating site.