Hi Sofia, yes you buy the licence for V3 BUT you do not install V3 — you install Scrivener 2.9 20th November 2017 from here:
literatureandlatte.com/scriv … S#release1
literatureandlatte.com/legacy-download
This version works with your Mac, and accepts the licence you will buy. Perhaps you can wait for someone from Literature and Latte to confirm, i am just another user, not a support person, but I think I am correct here. 8)