The certification assignment want you to use 3 tie.
How you do your work in practice is another question.
In real world application, you might now use EJB at all. The functionality of EJB can be replaced by plain Java class. So you have almost 2 tie architecture.
In UML design level, EJB = static methods.
2 tie / 3 tie there is no clear line in between.
I like use class then EJB, since if you only use Java code, IDE has wonderful tools like refactoring.
I always wonder how the hell to refactoring an EJB