“A guy walks into a bar…” is a typical form of what has been called the “bar joke.” A popular joke on the joke is:
 
“A priest, a rabbi, and a minister walk into a bar. The bartender says, ‘What is this, a joke?’”
 
The joke (which has many versions about the varied people who enter the bar) has been cited in print since at least 1999.
 
“A priest, a minister and a rabbi walk into a bar. Legally, bars in America have to serve people of all religions” and “A priest, a minister and a rabbit walk into a bar. The rabbit says, ‘I think I might be a typo’“ are other joke versions. “A priest, a minister, and a rabbit walk into a blood bank. The rabbit says,  ‘I think I might be type o’” is a related joke.
       
   
Forbidden Planet (August 21, 1999)
A black guy, a white guy, a Catholic priest, a Lutheran minister, a Rabbi, a hooker, a chicken, a football player, a baseball player, a wife, a husband, a Pole, a Russian, a little boy, a dog, a drunk, an Irishman, Saint Peter, and a penguin walk into a bar. Bartender says, “What is this, a joke?”
 
Google Groups: alt.music.stone-temple
new to the group
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12/1/99
(...)
i’ll leave my first post with a joke….......stop me if ya heard this 1 before….......a priest a nun and a rabbi walk into a bar and sit down….........the bartender walks over and says…........what the hell is this…...a joke?...........later folks…...........
     
Google Books
Mischief Marketing:
How the Rich, Famous & Successful Really Got Their Careers and Businesses Going (and how You Can, Too!)
By Ray Simon
Lincolnwood, IL: Contemporary Books
2000
Pg. 53:
Use storytelling formats. Example: “A rabbi, a nun, and an Irishman walk into a bar in New York City, the bartender takes one look at them and says, ‘What is this? A joke?’”
   
Google Books
The Reorganization:
The Sequel to Stolen Minds
By Robert L. Cunningham
San Jose, CA: Writer’s Showcase
2002
Pg. 232:
“Well,” Greg continued, “a priest, a nun, a rabbi, a lawyer and a doctor walk into a bar. The bartender takes one look at them and says, ‘What is this? A joke?’.”
 
Google Books
March 2003, Popular Mechanics, “Why We Laugh” by Jim Wilson, pg. 40:
A priest, a rabbi and a minister walk into a bar. The bartender asks: “What is this, a joke?”
 
Google Groups: rec.humor.jewish
Another Bar Joke? You try!
Marvin Margoshes
5/18/04
(...)
The bar joke that ends all bar jokes:
 
A priest, a minister and a rabbi walk into a bar. The bartender looks at them and says, “What is this, a joke?”
   
Google Books
The Reader’s Digest
Volume 169
2006
Pg. 115:
A priest, a rabbi and a minister walk into a bar. The bartender looks up and says, “What is this, a joke?”
 
Google Books
Inside Jokes:
Using Humor to Reverse-engineer the Mind
By Matthew M. Hurley, Daniel Clement Dennett and Reginald B. Adams
Cambridge, MA: MIT Press
2011
Pg. 138:
A priest, a rabbi and a nun walk into a bar, and the bartender asks them, “What is this, a joke?”
 
A Way with Words
What is This, a Joke?
Posted by grantbarrett on October 5, 2013
A priest, a rabbi, and a minister walk into a bar. The bartender says, what is this, a joke? Yep.
   
Reddit—Jokes
Posted by u/GaryCooperDraper October 24, 2018
A priest, a rabbi, and a monk walk into a bar…
Walks into a bar
The bartender goes; what is this a joke?
   
Reddit—Cleanjokes
Posted by u/GerryAttric May 2, 2019
A priest, a lawyer and a rabbi walk into a bar
The bartender asks “Is this a joke?”