This question is quite old; I hope this answer helps to document the new implementation or Nexus 2 "routing" in Nexus 3 "Content selector".
It's correct, Sonatype Nexus 2 "routing" capabilities have been substituted by "Content selector" in Nexus 3, based on JEXL queries.
Some notes are now available in Chapter 4 of Nexus Repository Manager 3.1 Documentation.
Basically you have to create a new selector from
Server Administration and Configuration -> Repository Content Selectors.
Define the JEXL query for your scope, e.g. the query below searchs for all path beginning with com/mycompany in maven2 repositories:
format == "maven2" && path =^ "com/mycompany/"
You can test your query using "Preview" buttun.
After that you go on more or less as in Nexus 2.
Server Administration and Configuration -> Security -> Privileges -> Create privilege
Give a name and description, select your "Content selector", select the repositories to apply the privilege and the action (comma separated list), e.g.
read,browse
Next create or modify a Role
Server Administration and Configuration -> Security -> Roles
giving the privilege you just configured.
Finally assign the role to the users you need.