??
I’ve heard this used mostly in reference to a couple of east-coast cities. Also “tri-city area”. Are these terms specific to certain regions? Here in CA, I heard a guy refer to Modesto, Fresno and Stockton as as “tri-city area”. Can these terms be bandied about in such a flagrant fashion?
I think it’s used by locals anywhere it applies. I’m from a tri-county area. But there are other tri-county areas that include my county as part of a different tri!
It could be anywhere an urbanized metropolitan area includes parts of three states. The Chicago metropolitan area includes parts of Wisconsin, Illinois and Indiana. The New York metro area includes parts of New York, New Jersey and Connecticut (and arguably Pennsylvania).
It depends where you are. If you’re in the NYC area, it’s NY, NJ, and Connecticut; in the Delaware Water Gap area, it’s NY, NJ, and Pennsylvania; in southern NJ (where ‘the city’ means Philadelphia, not New York,) it’s Pennsylvania, NJ, and Delaware. So yeah, bandy about as flagrantly as you want, as long as the three states are somewhere near each other.
In addition to the above-mentioned areas, there’s also the Cincinnati tri-state area, Ohio, Kentucky and Indiana.
The St. Louis metro area is frequently refered to as the bi-state area, though this is usually done by tv weatherpeople. It’s worse in Louisville where I’ve heard them call it “Kentuckiana.” I’m not sure of the spelling.
A little farther south in California and you have the TriCity Area of San Bernardino, Riverside and Redlands.
I used to work for a legal services company called “Tri-County Courier”; while based in Ventura county, we’d make deliveries of subpoenas and whatnot all over the place, as far away as Santa Barbara, Riverside, Victorville, even Bakersfield once. So whenever someone asked what three counties the “Tri-County” name signified, we’d say, “Ventura, Los Angeles, and…pick one!”
Then there’s the “quad corners” territory of Missouri, Kansas, Oklahoma and Arkansas; the tri-state “confluence” of the Mississippi and Ohio Rivers at Kentucky, Illinois and Missouri; the Gate territory of Iowa, Illinois and Missouri; and the grandaddy, the four corners of Arizona, New Mexico, Colorado and Utah.
In Memphis we have the Tri-state area of Tennessee, Arkansas, and Mississippi, though the area is much more commonly referred to as the Midsouth.
-Lil
La Crosse is a Tri-State area too, Wisconsin, Iowa and Minnesota.
(from our tallest Mississippi bluff you can see all three states at once, ooooh)
There is a Memphis TV station that in the 60’s advertised itself as “The Mid South’s something-or-other”. I was fascinated by the steamboat logo and the whistle. 
We drove from AR to TN to visit relatives a lot when I was a kid.