You can get 2 Wither Skeleton heads in the new tutorial level at X:308, Y:73, Z:194 and X:-319, Y:123, Z:-144. (thanks to SAVAGESANGHEILI)
For the 3rd head you need to kill a Wither Skeleton in the Nether.
You can enchant a sword to looting 3 to make it easier
(There's a couple villagers who sell it for 38 Emeralds and a book)
I got my 3rd Wither Skull easily by saving in a safe area by 2 Wither Skeletons. I just kept killing and reloading my save until I got a Wither Skull.
To see how to get all the achievements in the new Title Update check out my Achievement Review/Walkthrough