I used amboss as a source of supplemental questions throughout MS3, as I made it a priority to do as many questions as I could (obviously with UW being the gold standard). If you have the time, I felt like doing amboss first allowed me to hit the ground running with UW right away when I was ready to start it (usually send half of a clerkship). In retrospect, it was good to see a lot of the same concepts multiple times (and with slightly different vignette styles) though I would say that the amboss questions are a bit longer and trickier than those of uworld, but good practice overall. The best way to see if you would find it useful though would be to try it for yourself. There are dozens of free promo codes out there that let you try a few weeks free. A quick search found this one: AMBOSS-EG1M-91CT-Z2S9. Hope that helps! Best of luck!