I have found the three horcruxes (!). I have received many hints that I still need to burn the oak before having the quest solved by the Tzar. However, I can't seem to destroy the red oak where one of the items was hidden. Is this that needs to be destroyed a different oak?
Also, I haven't finished the game and this doubt may be clarified when I do, but at some point I forget which wizard character says that Kozneymort has five of them, whereas the Tzar's quest is for 3 items. Just curious.