Dungeon Siege III

Surprised there is not a guide to this, because you can mess it up. The biggest key to getting this is to not play multiplayer (online or "couch" co-op) using characters you haven't unlocked yet. If you do, then the companions will not appear when they should in the story, and you won't get this achievement without going back to a prior save or restarting the campaign.

Also, there is a side quest that you must pursue to get the third companion.

The order the companions appear is based on which particular main character you chose at the beginning of the campaign:

(1) Lucas: Anjali (will appear in a cage in cave after a mini-boss), Katarina (will appear on a bridge shortly after killing the first boss), Reinhart (in Stonebridge, during the Krug side quest)

(2) Anjali: Lucas (will appear in a cage in cave after a mini-boss) , Katarina (will appear on a bridge shortly after killing the first boss), Reinhart (in Stonebridge, during the Krug side quest)

(3) Reinhart: Anjali (will appear in a cage in cave after a mini-boss), Katarina (will appear on a bridge shortly after killing the first boss), Lucas (in Stonebridge, during the Krug side quest)

(4) Katarina: Anjali (will appear in a cage in cave after a mini-boss), Lucas (will appear on a bridge shortly after killing the first boss), Reinhart (in Stonebridge, during the Krug side quest)

The first two companions you acquire are very hard or impossible to miss (again if you are playing solo). To get the third companion, when you are in Stonebridge, get all of the quests from the Meisters. One of them will be to deal with the Krug in the prison in the same building; do this quick quest and the third companion will appear.

You will get this achievement after getting your last party member. If you have doubt whether you have messed this up, beat some monsters and look at the equipment that you pick up. If you get equipment for a companion before the key points in the story described above, the companion's portrait on the equipment should be blank. If you can see a portrait before you legitimately acquire the companion, you need to go back to an earlier save or restart from the beginning to get this achievement.

Edit: This achievement may still glitch on you, even if you don't play co-op. It really isn't known why. If you go all the above and the third character still does not appear, try and then play co-op and do the Krug quest to see if the third character appears. This happened to GameTagwastaken, and he was playing as Reinhart. One safer bet probably is to play as only Lucas until you get this achievement.

Please comment if you have any more information.

spacedvest2 did an awesome job for writing the solution.

I would just like to add a few things... since probably many of you will do same as me, play the game in co-op and not get that achievement.

First of, for any character you choose, the companions will be found in the same place.

1. companion - Quest:Search for the survivors - In a cage after a mini boss fight in Stormsong Cave (Anjali - unless you play with her)

2. companion - Quest:On the Road - After you beat Anjali (first real boss) , you will start your way to Stonebridge. As you come to the Bioluminescent cave there will be your second companion (Katarina - unless you play with her)

3. companion - Quest: Mediations (Majority Rules-Fiddlewick) - When you are in Stonebridge, and you get to Meisterhall, you will have to complete a side quest given to you by Fiddlewick: (dont miss this!) Mediations. You have to decide Krug's fate. After you do that , the third companion will join (Reinhardt - unless you play with him)

Now for the most important part. You will probably want to do it as fast as possible.
I suggest you just put the game on casual, and speed run it. Do just the Main quests, don't bother with the others.
Most of the time, you can just run away from the battle, cause it will end if you reach your quest marker (but keep in mind that you cant save while in battle).
I managed to get all companions in 1h 50min with Lucas.
It could probably be done faster if i wasn't fighting that much... i would say at least a half an hour faster.

Hope this helps clarify some things and that the tips for the speed run help.

This achievement is unmissable if done in solo.

However, if you are playing in cooperation, and if the other player is using a character at the precise moment of the story you're supposed to recruit him/her, you'll void the achievement.

If you want to be sure to earn this achievement and still play in coop, just be sure to recruit at first the character you want your teammate to use, and there will be no problem.