As I see it, the three places would be your header or footer, or on your about page. I see it as the same as a disclaimer that a post is using affiliate links. So maybe also as optional text on a particularly long form piece of text. Eg this comment of mine isn't long enough to warrant a disclaimer (though I did not use ChatGPT), but for something more longwinded, a maybe we're at a place we're a disclaimer makes sense. Maybe just pick one and feel it out. The disclaimer/disclosure good both ways though, I think. Telling people AI was used in the generation of the post cheapens it for anti-AI purists, but not everyone thinks that way. I personally am okay reading AI generated text, since I chat with ChatGPT on my own projects and for my own knowledge already, so I don't see why it would be so different encountering it in the wild, so to speak.

Okay, and now I've written enough that it turns out I feel like adding a disclaimer: this comment written by a human, with no help from an AI.