I used the OMT Review book by Severase for questions. There are 350 questions in the back of the book broken up into four tests (A-D). These questions were good for refreshing some of the obscure OMT concepts from 1st and 2nd year that most DO students forget, as well as keying in on the theory that is necessary for the test.
It is difficult to study for the questions you will encounter on the test. I felt Level 3 tested your cumulative knowledge base from 3rd and 4th year of med school, rather than the day-to-day handling of patients necessary to function competently as an intern.