To upgrade to Nexus 3, you must upgrade first to the latest Nexus 2.x version.
For more information, see Sonatype's documentation: Installation and Upgrades.
The following section describes how to migrate from an existing Talend Artifact Repository instance to a Talend Artifact Repository version that is compatible with a Talend product version 7.x or higher while keeping your configuration settings and the artifacts you have previously published to the repository.
For more information on the Nexus repository and how to use it with the Talend products, see https://help.talend.com/search/all?query=compatible+artifact+repository&content-lang=en-US.
The new Talend Artifact Repository configuration will be merged with your existing data at next start-up.