All three are correct, but they mean different things. "It" is just a general pronoun for talking about things in the third person. E.g. "Have you been to restaurant on Smith Street? It's the worst restaurant I've ever been to." "That" and "this" are similar, but they're directional. "This" is generally used to indicate you're talking about something you're holding or touching. If you're inside a restaurant, if you said "This is the worst restaurant..." there could be no doubt as to which restaurant you're talking about - the one you're in. "That" is the opposite of "this". You'd generally use it to refer to something you're pointing at with your finger. If you were walking down the street, you could say "That's the worst restaurant...", and 'that' would indicate you're talking about what you're gesturing at. Sometimes in speech you could use "this" to refer to something you're talking about, and you could use "that" to refer to something the person you're talking to is talking about (e.g. "This is a good idea" vs. "That's a good idea."), but in general if you're not physically using your hands to gesture towards something, use "it".