I think to use MTB properly one must do so with the videos and understand how to use MTB3 properly. MTB3 is not meant to replace Uworld or other sources. its not meant to have every word in there and every detail.
Its meant to teach you how to think like the test makers. ie. with CK you didn't need every dosing detail ie dilution of epinephrine in ACLS protocols or dosing of every single med, just the route and what the meds are. Or he breaks down how to read the questions for what details are important. If you can understand and put that in practice MTB3 with Uworld is good. but even with Uworld you must know which details are important vs. which aren't. I know for CK the important extraneous details not in MTB 3 were mostly pathophysiology questions and MTB3 didn't cover bone cancer tumors that well. This is where NBME subject and full length practice exams became key. For that matter even Uworld didn't cover all these topics completely thoroughly from what I saw.
But I cannot speak to step 3 yet except to say that if you are going to use MTB3 you must know how to utilize it and do so with conrad's videos where he goes into pathophysiology details or you will not get the most out of it and MTB leaves so much space so u can add in notes from uworld or kaplan qbank as well for that purpose.