My sonatype-Nexus-openshift repository is now able to deploy Nexus 3.x. You can check the README.md file to see how to deploy.

As the scripts will change the JVM memory configuration, it is possible to run Nexus 3.x on small gears.

Please keep in mind that at this moment is not possible to migrate Nexus 2.x to Nexus 3.x, so I suggest you deploy to a new cartrige.