I assume you are actually referring to the ROOT web-app that comes with Tomcat? This would be a ROOT.war (not ROOT.ear) in unexploded form as Tomcat does not process .ear files (which can contain, among other things, EJB's, which Tomcat does not process). To deploy this on JBoss 3.x, rename it to ROOT.war and copy the directory to %JBOSS_HOME%\server\[all|default]\deploy, where all or default is the server instance you are using. Use "all" if you start JBoss by "run -c all". Use default if you run Jboss via "run".
Access this webapp via http://localhost:8080/ROOT.
Hope this helps.