A triadic model of signs can be found by various researchers. Probably the most famous illustration is the diagram in Ogden and Richards's The Meaning of Meaning (page 11, digitized here). It is also attributed to Peirce as one of the two founders of semiotics, who writes:
The thing having this character I term a representamen, the mental effect, or thought, its interpretant, the thing for which it stands, its object.
The most common reference I could found is in the Collected Papers of Charles Sanders Peirce, volume 1, paragraph 564 (digitized here, often cited from 1932, but it seems to be published first in 1931). The original paper is On a New List of Categories, first published in 1867 in the Proceedings of the American Academy of Arts and Sciences (pages 287-298). This paper, however, does not include the triadic model, but Peirce added it later to the version printed in the collected papers. Peirce died in 1914, so obviously he has written it before, but where? People seem to just copy references without having looked for the original, don't they? ;-)