A straw man argument is one that misrepresents a position in order to make it appear weaker than it actually is, refutes this misrepresentation of the position, and then concludes that the real position has been refuted. This, of course, is a fallacy, because the position that has been claimed to be refuted is different to that which has actually been refuted; the real target of the argument is untouched by it.
(1) Trinitarianism holds that three equals one.
(2) Three does not equal one.
(3) Trinitarianism is false.
This is an example of a straw man argument because its first premise misrepresents trinitarianism, its second premise attacks this misrepresentation of trinitarianism, and its conclusion states that trinitarianism is false. Trinitarianism, of course, does not hold that three equals one, and so this argument demonstrates nothing concerning its truth.