They only work as a response to another comment, or as a beginning to a sentence. For example, "I think it necessary that..." Number (2) is wrong. In fact, you have a broken sentence. There's an interesting discussion on English forums as to why (1) and (3) do work. Perhaps it's because the pronoun "it" is used, and the rest of the sentence will define what "it" is. I'd also expect subjunctive grammar to follow, but that's just my impression. Pretty advanced stuff - make sure you get a handle on the more concrete parts of English before trying to comprehend this.

I agree with Peachey's answer below except I would add the following: Almost no one would use 1 or 3 in normal conversation. It soulds like a very old or 'written' style of grammar. The examples you have in parenthsis are what is used in normal conversation and you could even leave out the 'word 'that'. e.g. 'I think it is right' is close to the same meaning as 'I think that it is right'.