This was one of my father's favourites.
A chap gets talking to his new neighbour, and notices he wears a wedding ring, but hasn't seen the man's wife, so he gently enquires.
"I'm a widower. Been widowed three times."
"Goodness, that's awful for you, however did you come to have such misfortune?"
"Well, my first wife, she died of eating poisonous mushrooms".
"And what became of your second?"
"Oh, she also died of eating poisonous mushrooms".
"What a tragic coincidence! What happened to your third wife?"
"Oh, she died of a blow to the back of the head with a blunt instrument".
"However did such a terrible thing happen?"
"She wouldn't eat the mushrooms".