All language is contextual -- meaning various authors can define the symbols to mean something different than any reference tells you.
But, I have see the first being used as a true equivalences between sentences, and the third as a logical connective within a sentence.
So

would be a theorem (or axiom) connecting two sentences, while
 \leftrightarrow ( C \ne D ) ) \rightarrow ( (A - B)(C-D) = 0 ))
would be a single sentence, of the form "if just one of A = B and C = D is true, then (A - B)(C-D) = 0." With a theorem (or axiom) in a single sentence, you need to construct a syllogism (using something like modus pones) to apply it.