Completing an Instance

If say three players are at the same point in their personal story and one of them enters an instance with the other two and is successful, will all three players get the reward?

The instance opener will get the reward automatically. The others get a choice to either accept the reward and advance in the story with the same choices the opener made or refuse it and stay at their story step.

I am not sure if it is still that way but if you made different choices and are on another story path of the same levelin the past you could override your story path if you accected an other users story path achivementso you should be carefull about accepting it I guess as you wont be able to continue on your own path afterwards

If all 3 players are at the same story step and they do instance together then everyone will finish it and advance to next step.

If one of the players has no yet finish that story instance with that character then that player gets no rewards and has to do it again later to continue with the story.

Yet, if the actual question is about achievements you can earn in that instance, then these have no real logic. Sometimes everyone gets them, sometimes only instance owner, sometimes each player has to fill requirement separately.