Those released items (presumably that's' the practice CD you're referring to) are always a joke. It's frustrating that those are the Q's the NBME releases for free, when they're WAY easier than the real exams and representative of, essentially, nothing.
About buying the NBME practice test, here's me take: if you're willing to reschedule your exam, buying the NBME practice test is a great idea and can help you gauge where you stand. If things don't look so good, you can postpone the actual exam. However, if you're taking your Step 3 in 5 days rain or shine, I would not recommend buying an NBME practice test. The reason is a low (or failing) score will Psych you out going into your test. If you're taking Step 3 this week regardless, why risk getting a low score on the NBME practice test? Good luck!