"Tri-county area" (sometimes "tri-cities" or "tri-state area") is a universal description for a generic American region that's meant to be larger than a small town, but not a huge metro area. A tri-state area is somewhat smaller in landmass and population than regions like the US Eastern Seaboard or the British Home Counties (which border London). Think Everytown, America on a slightly larger scale.
This trope is Truth in Television for a couple of reasons. In the 18th and 19th centuries, the people who created American political jurisdictions had a tendency to use rivers as boundaries; those rivers also served as arteries of commerce, so that urban areas grew and expanded across political jurisdictions. In addition, on the East Coast of the US, every state began from initial colonies on the Atlantic Ocean, meaning that, initially, state boundaries were drawn so that states could have access to the Atlantic Ocean or the Gulf of Mexico, no matter how small (see: New York State, Pennsylvania, Alabama, and Mississippi's tiny coastlines).
People who live in these places frequently reside in one jurisdiction and work/attend school/socialize in another, forming a looser proximity community in addition to the existing cities. Later on, media outlets (newspapers, radio and local broadcast television stations) covered these proximity communities as their beats, in part because there were limiting factors, such as wealth, population and a small number of government-issued broadcast licenses. Combining neighbouring cities for social purposes also produces larger talent pools for things like sporting events (say, a tournament between teams from several high schools in the area) and cultural performances (band or choral contests, or forming an amateur theatrical company or symphony orchestra).
Sometimes this trope is used to mock the person using it as part of an Overly Narrow Superlative, including implications of a big fish in a small pond talking up the size of the pond. In reality, most of the "tri-state" references don't refer to the entirety of the three states involved, which underpins this mockery. Also, counties aren't generally very large, so even putting together three counties doesn't add up to a major population center.
The other major use of this trope arises from the practice of organizing emergency services around cities, counties and states. If something big threatens a proximity community (a fleeing felon or a marauding monster or a major storm), the authorities in multiple jurisdictions must be called and their actions coordinated. Invoking this trope can indicate the severity of the threat, much like the proverbial "five-alarm fire". If a blizzard hits, expect local news and weather reporters to say things like, "Downed power lines and limited visibility are affecting the entire tri-state area." In fictional works, characters may be depicted reacting to such local news reports, or they may use the phrase themselves when warning someone else, be it a loved one or a person in authority.
- At one point in Ultimate Spider-Man, after the Kingpin sees Spidey's face, one of his people points out that it would be impossible to find him, even if they searched all the teenage boys in the Tri-State Area alone.
- There's a Tri-Cities area in Pretty Cure Heavy Metal, where the Tri-Cities in question are Oriyama, Tsukimiya, and Isuten (in Japan).
- You Can't Cheat an Honest Man (1939):
I didn't get you the first time. I was one-time champion of the Tri-state league and the Lesser Antilles. Didn't know one card from the other when I started... but I stayed up at night marking with a pen.
- Ghostbusters (1984):
Good morning, I'm Roger Grimsby. Today, the entire Eastern Seaboard is alive with talk of incidents of paranormal activity. Alleged ghost sightings and related supernatural occurrences have been reported across the entire Tri-State area.
- Ghostbusters II:
- Have you been outside lately? Do you know how weird it is out there? We've taken our own head count. There seem to be three million completely miserable assholes living in the tri-state area.
- Crash Landing: The Rescue of Flight 232:
Sioux Tower, Rescue One, Roger your Alert Three. We're calling every unit in the tri-state area.
- Mystery Train:
Well now, you should buy this one here as well. The Tri-State Defender.
On June 17,1933, five of my finest men were ambushed escorting convicted bank robber Frank Nash to the Federal penitentiary. My men died like dogs in the gutter, and I swore personal vengeance. Mister Hoover told me he wanted these rats that did it exterminated, and that was my only job. Charles Arthur "Pretty Boy" Floyd. George "Machine Gun" Kelly". Lester "Baby Face" Nelson. Wilbur Underhill, "The Tri-state Terror". "Handsome Jack" Klutas. And, of course John Dillinger.
- Rain Man
You've been selected as a preliminary candidate to become our next Nielsen family in the tri-county area.
- Snakes on a Plane
I'm going to contact the National Poison Control Center to have them alert every ER in the tri-county area...
- Best in Show
I'm the chief hostage negotiator for the Akron and tri-county area.
- The Onion Movie
We've been the tri-county area's penis retrieval experts for 64 years.
Officials are calling for a complete and immediate evacuation of the entire tri-county area.
- In Garrison Keillor's Lake Wobegon Days he comments at one point that his composition class supplied the "three county area" with irony.
- The Mercy Thompson series mostly takes place in Tri-Cities, Washington.
- That '70s Show: In an It's a Wonderful Plot episode, Eric is the assistant manager of the second-largest mattress retailer in the tri-county area.
- Malcolm in the Middle refers to "the first Polish-American from the tri-county area to play three complete seasons in the majors." The boys enter Lois in the Miss Tri-County pageant.
- Buffy the Vampire Slayer: An incidental character in the first-season episode "Nightmares" once had the best spider collection in the tri-county area. It's unclear how this was determined...
- In one episode of The Dukes of Hazzard, Daisy enters the Miss Tri-Counties beauty pageant. This being Hazzard, one segment of the pageant requires the contestants to disassemble and reassemble a carburetor.
- Tim & Eric Awesome Show, Great Job! regularly introduces Jan and Wayne Skylar as "the only married news team in the tri-county area."
- In an episode of Perfect Strangers, Balki explains that Mypos and its neighbors Skeptos and Pathos are collectively known as the Tri-Island Area.
- The precursor of Eastern Championship Wrestling/Extreme Championship Wrestling was Philadelphia's Tri-State Wrestling Alliance (presumably the three states were Pennsylvania, New York, and New Jersey). The promotion's high point was probably the August 3, 1991, match between Cactus Jack and Eddie Gilbert; a two-out-of-three-falls affair in which the respective stipulations of the falls were "falls count anywhere", "stretcher," and "steel cage." The bout was third-runner-up for 1991's "Match of the Year" in Pro Wrestling Illustrated magazine (PWI being what it is, the match somehow got enough votes for fourth place despite occurring on an untelevised card in front of a crowd of 562 spectators).
- The Tri-Island Area is introduced in The Curse of Monkey Island, and mentioned thereafter. It consists of Mêleé , Booty and Plunder Islands, and their associated archipelagos, and Elaine is the hereditary governor of the lot of them.
- Parodied in an episode of King of the Hill by Peggy when she exclaims that the kids (Bobby, Connie and Joseph) aren't anywhere in the tri-home area (referring to the Hill, Soupanosaphone, & Gribble residences).
- Kim Possible lives in a tri-city area, Upperton, Middleton, and Lowerton.
- Phineas and Ferb parodies this with a tri-state area that's actually named from its founder, John P. Tristate. According to a history lesson sung by Love Handel, "The Tristate area used to be the Bistate area, with an adjacent area right over there" before the two areas were merged by John. This area belonging to Otto H. Adjacent, and apparently he didn't want this as his descendants, such as Buford, are offended at celebrating the merge.
- Tiny Toon Adventures: How I Spent My Vacation:
I want tri-state roadblocks! Call forensics and check the scene. I want dogs combing the woods! And some little sprinkly, candy chewies on a cupcake. NOW!
- Doug takes place in a "try-county area" [sic].
- Sofia the First's home of Enchancia is part of a tri-kingdom area with Wei-Ling and Khaldoun.
- The area around New York City—New York, New Jersey, and Connecticut—is commonly referred to as "the Tri-State Area". In this case, with a population of over 23 million and a gross metropolitan product of over $1.3 trillion, it's definitely not a small, provincial area (although that doesn't make its residents immune from provincialism).
- The area around Philadelphia is also sometimes called "the Tri-State Area" (Pennsylvania, New Jersey, Delaware). However, it is usually referred to as the Delaware Valley, probably because the above-mentioned New York one is literally next door and those living in New Jersey counties on the border might get confused (especially Mercer County - the US Census Bureau includes it in the New York Combined Statistical Area because of commuting patterns but its main television and radio feeds come from Philadelphia).
- Chicago's metropolitan area spills over from Illinois into Wisconsin to the north and Indiana to the east, though the region is more commonly known as Chicagoland. Again, this is probably because of New York.
- North Carolina has not one but two of these. The Research Triangle, or often just "The Triangle", is composed of Raleigh, Durham, and Chapel Hill (By no coincidence, they're also the homes of three of the state's largest and most important universities, North Carolina State University, Duke University, and the University of North Carolina at Chapel Hill). The Piedmont Triad, or "The Triad", is composed of Greensboro, Winston-Salem, and High Point note They're about ninety minutes down the road from each other via Interstate 40, and are therefore easily confused with each other by those who aren't familiar with the state. The Triad is to the west, closer to the mountains, and the Triangle is to the east, closer to the coast.
- Sharpsburg, North Carolina is a town where Wilson, Nash, and Edgecombe Counties meet. Part of the town is in each county.
- St. Cloud, Minnesota is also located where three counties meet - Stearns, Benton and Sherburne. The majority of the town is in Stearns County, with roughly 25% of the city split between the other two counties.
- Kennewick, Pasco, and Richland are "Tri-Cities" in south central Washington State.
- There's a Tri-Cities in Tennessee. The United States Census Board defines it to be the cities of Bristol, Kingsport, and Johnson City.
- The Quad Cities in the Upper Midwest, which are divided by the Mississippi River: Davenport and Bettendorf in Iowa, Rock Island and Moline/East Moline in Illinois. Effectively, there are at least five cities involved, with Bettendorf being the late up-and-comer; East Moline has its own government, and of course the suburbs reach to what were once physically separate communities. The area is rather densely populated, with something like 375,000 people. The local newspaper is The Quad City Times and there's also a symphony orchestra. The area was referred to as the "Tri-Cities" until the 1930s, when East Moline became more prominent.
- Sioux City is at the point where the state lines for South Dakota, Nebraska, and Iowa meet, at the junction of the Missouri and Big Sioux rivers. This results in Sioux City being divided into North Sioux City (South Dakota), South Sioux City (Nebraska), and Sioux City (Iowa).
- The Tri-County Council for Southern Maryland is the regional planning and development organization for the downstate Maryland counties of Calvert, Charles, and Saint Mary's.
- Wikipedia has this rather impressive list of tri-city areas.
- The heart of the Detroit Metropolitan Area consists of Wayne County (which contains Detroit) plus Oakland County and Macomb County to its north. As you might have guessed, it's called...
- The area around Pittsburgh consists of a tri-county area, depending on where you are; Beaver County, Butler County, and Allegheny County (which contains Pittsburgh) is a common one, but the Allegheny/Westmoreland duo-county area is generally referred to as well.
- The reason California's San Francisco Bay (including the major cities San Francisco, San Jose, and Oakland, as well as many smaller cities and suburbs, all spread over no fewer than six counties) is called the "San Francisco Bay Area"—but natives invariably call it just the "Bay Area", even when far from home, where people might not know which "bay" they're talking about.