“You put three Jews in a room you get four opinions.”
I have heard this phrase, or a variation of the phrase, used many times before. Sometimes the number of Jews in the figurative room varies, and sometimes you can get many more opinions (for example, sometimes it’s two Jews in a room and three opinions; sometimes it’s four Jews in a room and six or seven opinions.)
The joke relies on the stereotype that Jews are very opinionated people and suggests that if you have a certain amount of Jewish people in one room, you will get even more opinions than people that are in the room. I’ve only ever heard this joke told by Jewish people. The telling of this joke seems to be a way for Jews to reclaim a stereotype. There are a lot of less-than-positive stereotypes about Jewish people, some of which Jews spend a lot of time and energy actively refuting. Thus, the telling of this joke seems to be a way for Jews to acknowledge this particular stereotype and make fun of it, as if to say that it is all right to hold this belief because it is somewhat grounded in truth.