Digital Companion to C. S. Peirce
1897-8 | Abstracts of 8 Lectures | NEM 4:137

Let me remind you that three things do not necessarily make a genuine triad or have what I call threeness, except in a modified and reduced sense. By a genuine triad I mean three things in a genuine triple relation which is not a mere mixture of dual relations.

1903 [c.] | Logical Tracts. No. 2. On Existential Graphs, Euler's Diagrams, and Logical Algebra | CP 4.438

A rhema which has one blank is called a monad; a rhema of two blanks, a dyad; a rhema of three blanks, a triad; etc.

1906 [c.] | Prolegomena to an Apology for Pragmaticism | CP 1.292

In the present application, a medad must mean an indecomposable idea altogether severed logically from every other; a monad will mean an element which, except that it is thought as applying to some subject, has no other characters than those which are complete in it without any reference to anything else; a dyad will be an elementary idea of something that would possess such characters as it does possess relatively to something else but regardless of any third object of any category; a triad would be an elementary idea of something which should be such as it were relatively to two others in different ways, but regardless of any fourth; and so on.