In Chapter 16.2 of the Sonatype books for Nexus 3 it is stated that you can start the Karaf console by pressing enter after the nexus run command:
Bundles can be loaded via the Karaf console. To enable the console, set karaf.startLocalConsole in bin\nexus.vmoptions to true. This allows you to access the Karaf console by pressing enter after starting the repository manager with the run option.
However doing so results in a
java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at com.exe4j.runtime.WinLauncher$1.run(WinLauncher.java:80)
and the system is shutting down immediately. Turning on the Karaf debug logging does not reveal any further hints.