You need to upgrade your Nexus to the latest Nexus 2.x version as detailed in Upgrading from Nexus 2.x to the latest Nexus 2.x version available before starting the upgrade to Nexus 3 as detailed in Upgrading from Nexus 2.14.8 and beyond to Nexus 3.
For more information, see Sonatype's documentation: https://help.sonatype.com/repomanager3/upgrading.
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 Compatible artifact repository.
The new Talend Artifact Repository configuration will be merged with your existing data at next start-up.