The Comfort of Strangers To get this achievement you have to reach the beginning of act 2, once you meet the Enchantress and complete the quest with her she will give you an option that she will join you. There can only be 1 companion following you so if you accept the companion that already is with you will go back to town. Don't worry if you ever decline them to join you, they are always in the town of what ever act you are on.
To get this achievement you just have to have all of them join you at 1 point. This is missable as you can decline them to follow you, but you can always get this achievement if you are already past the first area in Act 2, you can just go into town and recruit all of them by talking to them(A button) then Y for them to join, just do it for all 3. Once all 3 have joined you separately you will unlock the achievement.
Templar- Right before fighting the first actual boss
Scoundrel- Right after saving some people from bandits
Enchantress- Right in the beginning of act 2, finish the quest with her and then you gain her as a companion
NOTE: If you are importing characters simply walk up to them Hit A, then Y on all 3 of them.