I would recommend not trying to farm or force this. Everyone will without a doubt achieve this in natural playthrough of the game.
For me this unlocked during ACT II when you go north from Kyovashad, tons of all three enemy types will be in your way.
The exact moment of unlock will always vary since you can encounter enemies in dungeons and side missions that will add to the count as well.
Just play, have fun and will unlock on its own!
