I like Ciel has it down pretty well...
Cuckolding is about the humiliation of watching your SO with someone who is supposed to be able to satisfy better than you are. So, a man watching his wife with a guy with a much larger penis. The point is that you are humiliated.
A triad or even a vee in a poly situation shouldn't be about humiliation. Poly is a loving connection between multiple people. Could cuckolding be a part of a poly situation? Yes, but in a traditional cuck relationship, the third person (the second guy in the example) is only involved sexually, not romantically. So, while they could overlap, I don't think they are inherently connected at all.