http://grails.github.io/grails-doc/3.0.x/guide/upgrading.html
Follow above instructions in order to get the content of your grails plugin from older grails into grails 3.
A few notes on plugin names and folder names, I put down a few things here: https://groups.google.com/forum/#!topic/grails-dev-discuss/cG_6mvAO3f8
In there I mentioed mvn install - which does not appear to work under normal circumstances, in order to get a plugin to work locally follow grails install instructions or do it as inline plugin ..