Here is a good collection of info about the exam Oracle Exadata Database Machine X3 Administration: https://blogs.oracle.com/certification/entry/0873
It will surely help to attend the Oracle University course mentioned there. Furthermore, go through the documentation supplied on the Exadata Database Machine.
A very good 3rd party book is Expert Oracle Exadata by Kerry Osborne, Randy Johnson & Tanel Poder
You may also find my collection of postings about Exadata helpful: Exadata | The Oracle Instructor
You can take the Exadata Workshop: http://education.oracle.com/pls/web_prod-plq-dad/db_pages.getCourseDesc?dc=D73668GC10
and the links in the next article: Oracle desde Guatemala: &#191;Cómo Prepararse para el Examen de Exadata?
(the blog is in spanish but the links are in english)