“Any time three/four New Yorkers get into a cab/taxi without an argument, a bank has just been robbed” is a joke showing that New Yorkers argue in the back of taxis. The joke has been credited to late night talk show host Johnny Carson (1925-2005) and also to comedienne Phyllis Diller (1917-2012), who was a frequent guest on Csrson’s The Tonight Show program. The joke has been cited in print since at least 1988.
 
 
17 July 1988, Augusta (GA) Chronicle, “Postcard collection a big hit” by Bill Kirby, pg. B1, col. 1:
Anytime four New Yorkers get into a cab together without arguing, a bank robbery has just taken place.
—Johnny Carson
 
Google Books

A compendium of over 1,000 great jokes from today’s hottest comedians
New York, NY: Cader Books; Kansas, City, MO: Andrews and McMeel
1996
Pg. 156:
Anytime four New Yorkers get into a cab together without arguing, a bank robbery has just taken place.—Johnny Carson
   
Google Books
A Treasury of Humor:
Thousands of humorous stories, jokes, anecdotes and one-liners, up-to-date and categorized for ease of use
By Lowell D. Streiker
Peabody, MA : Hendrickson
2000
Pg. 287:
Any time four New Yorkers get into a cab without arguing, a bank robbery has just taken place. —Johnny Carson
 
Google News Archive
12 March 1997, Catoosa County News (Ringgold, GA), “Take a look at some of those books in your den” by Richard L. Ball, pg. 4A, col. 4:
Anytime four New Yorkers get into a cab together without arguing, a bank robbery has just taken place Johnny Carson
 
2 May 2002, Associated Press, “Some of Phyllis Diller’s one-liners”:
“Any time three New Yorkers get into a cab without an argument, a bank has just been robbed.”
 
New York (NY) Times
Johnny Carson’s Long Symbiosis With New York
By SAM ROBERTS
Published: January 26, 2005
(...)
That New York - the New York of blackouts, daylight muggings, noisy middle-of-the-night street repairs and, of course, cabbies (“Any time four New Yorkers get into a cab together without arguing, a bank robbery has just taken place”) - was also the butt of Mr. Carson’s monologues wasn’t beside the point.
 
Google Books
The Encyclopedia of New York City: Second Edition
Edited by Kenneth T. Jackson, Lisa Keller and Nancy Flood
New Haven, CT: Yale University Press
2010
Pg. ?:
On the show, the city was often the butt of his monologues; he famously joked, “Anytime four New Yorkers get into a cab together without arguing, a bank robbery has just taken place.”
(Entry for “Johnny Carson”—ed.)
 
The Daily Beast
Phyllis Diller Dies at 95: The Comedian’s 20 Funniest Zingers
Aug 21, 2012 4:45 AM EDT
(...)
12. Any time three New Yorkers get into a cab without an argument, a bank has just been robbed.