When I was a kid, my father told me there were three things to never bring up in a crowd: Religion, politics and I can’t remember the third. Does anybody know?
Sex.
The Great Pumpkin.
money
Never bring up the water table in a crowd.
Never bring up your last meal.
Seriously, I would say sex. It’s not necessarily that these three topics will generate heated discussions (though in some cases, religion and politics certainly could), but that they tend to very easily put people in awkward positions that make polite discourse difficult.
Ask Opal?
Circumcision.
Planes & Treadmills
Babby.
What to call those little peanut-butter-and-chocolate ball-shaped treats.
I think this is one of those “fill in the blank” phrases. As an IT student, I often heard it told as “Three things to never bring up in a crowd: Religion, politics and operating systems.”
I’m not sure there’s an official third thing. But money is probably the most common.
The declawing of cats?
Sex
As I recall the usage of the saying, it was “religion, politics, and …” whatever topic had just been bought up, that was causing an argument. Sort of the final word on the topic at hand, it is offically too hot to handle. Kinda like when someone Godwin’s an internet thread.
No, sex with your mother; always a lively topic!
My wife’s family is Egyptian. Many years ago an American told her dad that when you first meet an American, you shouldn’t talk about religion, politics, or ask them what their job is.* He said, “What else is there?” 
*In old school etiquette it is considered bad form to ask someone what their job is, because it implies you are evaluating whether they are of a high enough social class to bother with. I live in the D.C. area and the atmosphere here is such that it’s almost always the first thing people ask when they meet you. Once I was actually at a party when I met a woman, the first thing she asked was my job, the second thing she asked was my salary. It turned out she was a technical recruiter, and I guess those people are thinking about who they can recruit 24/7.
I emailed your dad. He doesn’t remember exactly what he said to you, but he says at your house it was “Religion, politics and That Time You Wet Yourself While Playing a Sheep In The Christmas Service”.
abortion - it’s like bringing up politics, religion, and sex all at once; with the added bonus of dead babies