The USMLE Step 2 CK section of content in Firecracker is very relevant in its coverage of materials that are fair game to be tested on USMLE Step 3 (we estimate that our Step 2 CK section is 80%+ comprehensive in content coverage for Step 3 testable subjects). I would encourage you to mark topics and do practice questions according to your subject area strengths and weaknesses.
To accurately gauge which subject areas you should study more, I would recommend that you take Firecracker's USMLE Step 2 Practice Exam, and/or the 100-question Shelf-style practice exams we offer for Emergency Medicine, Family Medicine, Internal Medicine, Pediatrics, Psychiatry, and Surgery (you can find the timed practice exams under our Take an Exam page): https://med.firecracker.me/test#/exams
While we don't have a Step 3-branded practice exam at this time, the Step 2 Practice Exam is definitely relevant and will serve not only as good practice, but also as a measure of your preparedness for your upcoming board exam. Once you complete each practice exam in Firecracker, you will receive an in-depth breakdown of your results, which includes comparative performance metrics and topic-by-topic Refresh exercises under the "Next Steps" tab. Your personalized Refresh exercises are based off any of your incorrectly answered questions from the practice exam, and will help remove the guesswork regarding which content areas you should focus on for your review and remediation efforts.