No: in your first two examples, the that– clauses (or more correctly 'content clauses') are complements of the verbs "knew" and "heard", and in your third example, it’s the subject of the sentence.
Content clauses serve to expand, or complete, the meaning of the item they complement. So in you first example, the clause explains what it was that you knew was true, and in your second example it explains what it was that you heard. In some respects they are rather like objects, cf. "I knew the truth", hence their often being called noun clauses. In your third example, the clause states what it was that was well-known.
I heard that they left simply means that you became aware of their departure, whereas I heard their leaving means you physically heard the noise(s) they made as they left.
In case you're interested, content clauses almost always have a complement function. They are the 'default' kind of finite subordinate clause lacking the special properties of relative and comparative clauses in that they are selected purely for their semantic content (hence their name). They are most often introduced by the subordinator "that", but the rest of the clause does not usually differ from that of a main clause.