Nexus Repository Manager 3 does not have anything equivalent to Smart Proxy so you’d have to write a script to move artifacts between the two instances. We have added a new feature (for pro customers) to import maven content from 2 to 3 so long as the content is on a filesystem that repository manager has access to. We’ll be adding additional formats as well as the ability to export data in the future. Longer term we’re looking at ways to provide a better replication story but I have no idea when that will happen.