“When you point your finger at someone, three are pointing back at you” refers to the index finger and the three other fingers exclusive of the thumb.The saying means that when you accuse someone of fault, be careful that you’re not the one really at fault. The saying has been used in business, in politics and in other fields.
 
The syndicated newspaper columnist Bennett Cerf credited American journalist John Gunther (1901-1970) with the saying in 1945:
 
“John Gunther reminds you that every time you point your finger at somebody, you point three fingers at yourself at the same time. I just tried it, and by cricky, he’s right!”
 
 
7 May 1945, Lowell (MA) Sun, “Sampascoopies” by Charles B. Sampas, pg. 5, col. 1:
Bennett Cerf points out: “John Gunther reminds you that every time you point your finger at somebody, you point three fingers at yourself at the same lime.” Tr it and see.
 
29 May 1945, Morning World-Herald (Omaha, NE), “Tripling It” (Bennett Cerf in Omnibook), pg. 6, col. 3:
John Gunther reminds you that every time you point your finger at somebody, you point three fingers at yourself at the same time. I just tried it, and by cricky, he’s right!
 
Google Books
A Guide to Confident Living
By Norman Vincent Peale
Greenwich, CT: Fawcett
1948
Pg. 180:
“Point your finger at me,” he said. I did so. “Now, what are the other three fingers doing? Pointing back at you, aren’t they? You see I win three to one.”
 
22 May 1963, The Times-Picayune (New Orleans, LA), “Tully Elected by N. O. Ad Club,” sec. 3, pg. 16, col. 1:
“But remember this,” he (Mark F. Cooper, president of the Advertising Federation of America—ed.) added, “when you point your finger at Washington, three of your fingers are pointing at you.”
 
Google Books
Healing of Memories
By Matthew Linn
Mahwah, NJ: Paulist Press
1974
Pg. 35:
The Nigerians graphically say, “When you point a finger at another, there are three fingers pointing back at you.”
 
Google News Archive
6 November 1974, Prescott (AZ) Courier, Letters, pg. 4, col. 1:
There is an old adage, it is “when you point your finger at someone, three are pointing back at you.”
Sincerely,
Margaret E. Stevens
 
Google Books
The Speaker’s Quote Book:
Over 5,000 Illustrations and Quotations for All Occasions
By Roy B. Zuck
Grand Rapids, MI: Kregel Academic & Professional
2009
Pg. 123:
When you point your finger accusingly at someone else, remember you have three fingers pointing at you.