Fallacy arguments

Okay I am an amateur of philosophy I've studied a little Sartre. I know A little Plato and I'm stuying all sorts of theories of moral philosophy right now. I'm also really interested in Aristotle's "natural law" theory.

So can any of you philosophical minds tell me a little more about the fallacy argument. I know if A=B and B=C then A=C. Is this also represented with a VENN DIAGRAM? You know the one with the bubbles that represent a certain category. It would be very helpful.
THank
JAne