You get this not after saving each but after making each your follower (you don't need to do anything with the follower, just add them). The soonest you can get this is after you save the Enchantress at the beginning of Act 2.
The achievement will only pop after you have made them follow you outside of the story of their character.
Note: This trophy likely is not able to be gained in multiplayer (as you don't have followers) but your multiplayer progress can allow you to unlock this three followers and when you go back to single player just have each one follow you for a moment).
Note: This trophy is NOT missable, all three have to be recruited as part of the game. And none of them disappear.