Was the poison ingested, or was it strong enough to kill just if you got it on you?
Theory for poison administration:
Perhaps all three were in on the murder? One of them could have sneaked into the building and put poison on one side of a knife, then wiped off the excess with the napkin. All three could be lying that they were there the whole time. The murderer would see that they had silverware, and just ask them if they needed any to throw the police off the trail. The knife would be on the table, so maybe some poison would have gotten on it. When the woman cut some chicken or something, the poison got in her food.
Just a crazy theory.