Docker Application Portability to Openshift

Is docker supported early or live, or will it be in a future V3 release? If so, how does it work?

We just released the V3 repository, so it's at Origin https://github.com/openshift/origin .

I saw there are different docker images available here to open in docker hub. How it works?

You answered this in the comments, just clarifying here for visibility.
Linux containers or "Gears" have always been the main component of OpenShift. Docker is the new standard for containerization through the libcontainer project. To quote this great article https://www.openshift.com/blogs/openshift-v3-platform-combines-docker-kubernetes-atomic-and-more :

The OpenShift v3 cartridge format will adopt the Docker packaging model and will allow users to use any application component packaged as a Docker image. This will allow developers to connect to the Docker Hub community to access and share container images for use in OpenShift. Customers will also be able to use images of Red Hat certified containers from both Red Hat and our ISV partners. Our newly opened OpenShift Marketplace will expand to include solutions from both SaaS partners and Certified ISVs. ......
In OpenShift v3, we will be integrating Kubernetes into OpenShift Broker to manage container orchestration.

Docker is mainly for application portability, lets say I already have an application running on my docker installed locally, so how can it be ported to an open-in environment, or how can I create images from the open docker?

I am citing the article again, but "The OpenShift v3 Cartridge format will adopt the Docker packaging model and will allow users to use any application component packaged as a Docker image. This will allow developers to connect to the Docker Hub community to access and share container images for use in OpenShift."