My husband has threesome fantasy

I am 23 year old married women.mu husband told me two years back that he wants me and my mother for threesome.I was shocked to listen but then he told me that he is physically attracted to my mother.He has this fantasy of having us together since we came into relationship.I told him that I haven't thought about my family like this way before and it's impossible to do because my family is not that open.He told me atleast I can make my mind then he will find a way to do this.zhe is telling me it's normal to share and it's happening All around the world and take time and join community in internet.I am not really sure what should I do