Concepts for Peirce are mental symbols, so they fall under the general designation of signs. For triadic sign relations in general, then, we are considering a triadic relation among objects of signs, signs of objects, and what Peirce calls interpretant signs, or interpretants for short. It is critical to regard the designations of objects, signs, and interpretants as relational roles not ontological essences. It is also critical to distinguish (a) extended sign relations, (b) elementary sign relations, (c) the slots of an ordered triple, and (d) the things that fill those slots.
Triangles like the one linked above have long been used to introduce the idea of a triadic sign relation. They have the unintended consequence, however, of leading people to miss all the points I mentioned above. So it’s wise to move quickly on to better pictures and more detailed descriptions.