The root of all your questions is really why does Sync Gateway think my-db is defined in DataBucket-3 instead of DataBucket-1.
I suspect what’s happened is you have somehow got into the situation where your Couchbase Server buckets have multiple Sync Gateway databases with the same name and Sync Gateway is choosing the first one it finds.
Can you clarify what buckets you have, and what Sync Gateway databases you expect to have created on each?
If you don’t want Sync Gateway looking in other buckets, you can restrict the RBAC permissions used by Sync Gateway’s bootstrap user, or specify a set of bucket_credentials