Denver / Salt Lake City / Calgary (Intermountain West cities.)
Atlanta / Charlotte / Birmingham (Piedmont area.)
Las Vegas / Phoenix / Albuquerque (Southwestern cities.)
Reno / Boise / Spokane (Mid-sized cities near small mountains or hills.)
Flagstaff / Casper / Santa Fe /(4th) Rapid City (Small cities near mountain features.)
Milwaukee / Chicago / Cleveland (Great Lakes.)
Indianapolis / Columbus / Kansas City (Midwest cities with a loop.)
Seattle / Portland / Boston (Liberal, northern cities on or near the ocean.)
Seattle / Portland / Vancouver (Cascadian cities.)
Seattle / Vancouver / San Francisco (Dunno how to explain without using the phrase "West Coast" because LA is also on the West Coast but I'm sure you get the point.)
Miami / Tampa / San Diego (Tropical cities on the waterfront.)
Oklahoma City / Wichita / Omaha (Plains cities on the border between the treeless plains and tree-ful(?) plains.)
The above can also be Oklahoma City / Fort Worth / San Antonio. That general string of cities.
Tucson / Colorado Springs / Salem (Large but smaller cities overshadowed by the larger one in close proximity.)
Savannah / Charleston / New Orleans (Southern cities with rich history.)
Amarillo / Lubbock / Midland ([Naturally] Treeless cities on the plains, flat as a pancake.)
I'll just stop now...