You know I have no idea why that area is referred to as tri state and never really thought about it either. I know it’s something they used more often decades ago, when the quickest way north to the white mountains was through Rochester so maybe tri state refers to Mass traffic? I tried to look this up and couldn’t find an answer.

edit:

or maybe tri state refers to the three towns as they were a largely populated area of NH in the past with economic growth seeing as major roads and highways used to pass directly through them. (obviously not anymore)