That quest was designed so that you could only cure two of them. However, you can Pickpocket (level 2) a cure potion back, or kill one of the cured people (no reputation loss) to get a potion for the third.
There is a letter to Elrath in the cellar implicating him in the poisonings. If you leave the house with it, you will trigger the final cut scene with Elrath and be able to complete the quest.