I've started helping with a catechism class of fourth/fifth graders at my church. That was the question with which Liz began the most recent lesson. I thought it was kinda basic, until a precocious student in the front row yelled out, "Three!"
I found out afterwards that in RCIA class (which is for adults who are becoming Catholic), Fr. John asks the same question, and he usually catches someone giving the same wrong answer.
The Holy Trinity: principal mystery of our faith. Apparently it's easy to trip people up with this. Try it today!