They all are correct.
Consider the following sentences -
- How you say that is funny.
- What you did is amazing.
- What happened is terrible.
These sentences make sense and indeed correct.
Consider sentence #2
What you did is amazing.
The bold part - what you did - is either a fused relative construction or an open interrogative.
Consider sentence #3
What happened is terrible.
The bold part - what happened - is either a fused relative construction or an open interrogative.
Consider sentence #1
How you say that is funny.
The bold part - how you say that - is an open interrogative, not a fused relative clause.
A fused relative construction can never occur in extraposition construction. But an open interrogative can.
The following sentences are correct -
It's possible that out team might win.
Here that is a subordinator and it introduces a subordinate clause to complete the meaning.
It's I who did it.
In this sentence who did it is a relative construction and who a relativizer. The relative construction is adding some information about its antecedent - I.