As a European, I'm confused and would love to know more about this specific area.

As an American, I’m equally confused. I will say that based on my experience, the tristate areas I often see referred to are around New York and Philadelphia.

New York: New York, New Jersey, Connecticut.

Philadelphia: Pennsylvania, New Jersey, Delaware.

Tristate is used in things like radio where you are covering 3 states. "Here is the news/weather in the tristate area". They mean specifically the 3 states that the radio signal reaches, which is clear to everyone. Since they reach 3 states there is no other name that can be used for their coverage area. The people in question regularly cross state lines.

It isn't a useful concept to bring up when you are not physically in a tri-state area.

And if you're from a little-further-south it is (naturally) Washington DC's version of the same thing (Maryland, Virginia, West Virginia)

I guess that’s true if you’re west of DC. Otherwise it’s Delmarva (DELaware, MARyland, VirginiA).

I’ve only heard that called “the DMV,” but I’m not from there.

Chicago: Illinois, Indiana, Wisconsin

Much like residents of the bay area [0], I'm playfully suggesting there's only one tri-state area of significance [1].

[0] https://en.wikipedia.org/wiki/San_Francisco_Bay_Area

[1] https://en.wikipedia.org/wiki/New_York_metropolitan_area

California has three regions known as the 'South Bay'.

https://en.wikipedia.org/wiki/South_Bay_(Los_Angeles_County)

https://en.wikipedia.org/wiki/South_Bay_(San_Francisco_Bay_A...

https://en.wikipedia.org/wiki/South_Bay_(San_Diego_County)

Solid, liquid, gas?

It is video related, so plasma could be valid as well

Everyone being pedantic would change their tune instantly if it were an even money family-feud style bet for the top answer.