This is still a haunting question in semantic and semiotic theory. That said, two might be the number of symbols (difference), but three is the number of signs. Another reason that I think three is the key number is because signs are not about one-to-one relationships between terms (that's a symbol!) Signs are about multiplicity and ambiguity; a sign has connotations and ambiguities, it can be used not only to name but to give hints and even lies. You need at least three nodes in a graph (in layman's terms, network) to create something more than a simple one-to-one mapping.
2) Speaking of signs and triads, here's a quote that I found from The Name of the Rose that seems to implicitly talk about the triad of signifying/inferring/lying that I talked about earlier:
"It is of use to me as Venantius's prints in the snow were of use after he was dragged to the pigs' tub. The unicorn of the books is like a print. If the print exists, there must have existed something whose print it is. ...The idea is sign of things and the image is sign of the idea, sign of a sign. But from the image I reconstruct, if not hte body, the idea that others had of it." -William of Baskerville