Of the three, the best answer is "Yes, he is." Since the question is "Do you think," a better-fitting answer would be "Yes, I do." Here are some questions and matching answers. "Does Santa exist?" "Yes, he does." "Does Santa live at the North Pole?" "Yes, he does." "Is Santa real?" "Yes, he is." "Do you think Santa is real?" "Yes, I do." "Do I look like Santa Claus?" "Yes, you do."

Errm, all could be answers to different questions. But 'Do you think Santa is real?' could be answered by 'Yes, I do,' or simply 'Yes.' If you're asking me, then the answer is: 'Yes, I do.'' ;-) And if you see him, please ask him to park the new Ferrari in my drive and to put the keys into my letterbox. :-)

Yes, he is.

Of course, he is real! ;)