In a relative clause introduced by a relative pronoun, the pronoun substitutes for one of the main parts of the clause, either the subject or a complement of the verb. Let's look at these three relative clauses by "undoing" the substitution. We'll use This for the part that which stands for:
- which is what I am a part of ...
Substituting this for which gives us This is what I am a part of = What I am a part of is this. That's a perfectly OK sentence, so this is grammatical.
- which is I'm a part of ...
Substituting this for which gives us This is I am a part of. A part of what? Of this? Moving this to the end gives us is I am a part of this. This doesn’t work: there’s an extra verb on the front. So this version is not grammatical.
- which I'm a part of ...
Substituting this for which gives us This I am a part of = I am a part of this. Once more, the sentence is OK, so this is grammatical.
So 1 and 3 are both grammatical. Which one is “correct”?—that depends on what you’re trying to say. The purely propositional content of both is the same—“I am a part of this”—but they focus on different things, and arise in different contexts:
- Which I’m part of arises when your interlocutor is asking about the plan. It gives new information about the plan: “It’s not just your plan, it’s also the plan I’m part of.”
- Which is what I’m part of arises when your interlocutor is asking about you:
HIM: Tell me, dibyendu, what is it you do? What are you part of?”
YOU: Well, sir, this!
HIM: What’s this?
YOU: It’s Your Plan, sir!
HIM: My plan?
YOU: This is Your Plan, which is what I’m a part of! And very proud to be so!
HIM: Oh. Well, dibyendu, it does you credit!
YOU: Thank you, sir!
Whichever version suits, a comma should follow plan
; bare This is your plan
implies that your interlocutor has only one plan. For either clause to be restrictive and not require the comma you would have to say This is the plan of yours which &c