There isn’t a law of logic here, because neither the conclusion |3|=3 nor the introduction of anything else justifies it.
The first clause has the following structure: For all x, if A(x), then B(x).
The second statement would be an example of Modus ponens, preceded by Universal instantiation, if it said "|3|=3, hence 3 is positive."
Instead, the second clause states, "x=3 so A(3)." We could read that as an attempt to assert the consequent by saying, "3 is positive, thus A(3)," but that would be a fallacy. To express "if and only if," we would need to make the first sentence stronger.
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