From what I can see there is no need to have V3.x. We can cover most (if not all use cases) with version V2.x.
If you have a look at Docker documentation Release V3 Compose the V3.x introduces features that are mainly focused on Docker Swarm (similar to Kubernetes). How we designed the platform now is that each device works on its own therefore it does not make sense to upgrade to this version because we are not using Docker Swarm. That would be the short answer I would say.