1. How the heck do people find these threads? I can understand it if the thread is still on the first page, but three years old? How the heck does that work out.

Some people have the postdates turned off so they don't realize how old a thread might be, especially if they do a search or look in Who's Online.

2. How is it that people will continue the conversation as if nothing was out of the norm after a thread has been bumped months or, in some cases, years after said conversation ended.

Some (most) people never look at postdates and just post.

3. Why do threads that are three years old still exist in the first place?

For reference, reading entertainment, etc., etc.