This depends on what your need. Storage groups are basically a source volume and one or more destination volumes. If your trying to sync appstacks between all 3, if you pick a primary one, have that one sync to a nfs data store. Then In each of the other vcenters have the data stores there sync from the nfs share. So You basically need one per vcenter unless your trying to appstacks at different locations. The only thing I'd be careful of is creating a circle, when vc1 replicates to vc2, then vc2 replicated to vc3, and vc1 replicates back to vc1. I'm not sure appvolumes will handle that appropriately, but maybe someone else had tried it.
I'm basically trying to spread out writable volume creation. Currently we have a writable volume assignment that points to a storage group so that when a new user logs in they will get a new writable created on one of the datastores in the storage group. Now we have expanded and have 3 vCenters, so trying to make a similar mechanism, but was unsure if I could do it with one storage group or if we needed 3.