It must be still a package or coding problem.
I use ejb3 without any descriptor except application.xml
The structure is
META-INF/MANIFEST.MF
META-INF/application.xml
ejb.jar # ejb3 stateless session bean and interface
persistence.jar # JPA entity used by the SLSB
application.xml:
<application>
<display-name/>
<description/>
<module><ejb>ejb.jar</ejb></module>
<module><java>persistence.jar</java></module>
</application>
In my example the ejb.jar include Bean and Interface. If you have a seperate interface jar you can add it like the persistence.jar.