I've been wondering what can replace a relative pronoun 'what' like the sentence below. Some grammar books says relative pronoun 'what' is as same as 'the thing which' while others say 'the thing that'
I've heard that if a precedent is a word finished with -thing, -body, and -one. you can put a relative pronoun 'which'.
1. This is what I've been looking for.
2. This is the thing which I've been looking for.
3. This is the thing that I've been looking for.
So, which is grammatical between 2 and 3?