Pertaining to Ind.-Ill.-Mich., e.g.

• Around a geographical meeting point
• Conjoined area
• Pertaining to Pennsylvania, Ohio and West Virginia, e.g.
• Like the New York metropolitan area
• Pertaining to some greater metropolitan areas
• Like the ArkLaMiss area
• Involving New York, New Jersey, and Connecticut, for example

Wiktionary Word definitions in Wiktionary
a. 1 Having, or pertaining to, three distinct states. 2 Of, pertaining to, or characteristic of a region in the United States where three state borders come to a single point. n. The area in some regions of the United States where three state borders come ...