This is what I think

Yes, I agree with Onceuponatime and NewVoice, snd I think all of you three (Hammerheadshark, 非文学青年, and the native speaker) are correct.

1. "Excuse me, but is this the dentist's office?" implies that the narrator no longer thinks it is the dentist office he thought it was (after hearing what the Chinese said). This can still serve as a good punch line.

2. "Excuse me, but is this a dentist's office?" implies that this Chinese has turned this dentist office into an immigration office. This is where the puch is.

3. "Excuse me, but is this dentist's office?" means the same thing as 2, but the narrator speaks ungrammatical English here.