Three Pieces in the Attack As a kid, my coach, an International Master, was repeating "if you have three pieces or more in the attack you can sacrifice without thinking!" 

However, don't take this rule too literally!


Chess is a concrete game and we should not follow a rule "without thinking". However, I believe that chess is guided by some rules and general principles.
We can redefine the rule of "Three Pieces in the Attack" as "Play more aggressively in the attack" in a context where the number of pieces attacking exceeds the number of pieces defending.


My question is: by searching on the Internet, I could see this rule from time to time but I could not find its origin. Who devised this rule? How old is it?

I found a cute example here to illustrate this rule. White plays and checkmates a strong GM!

[title "Juarez Flores, C - Lputian, S, Manilla 1990"]
[fen "r6r/p4ppp/2p1b3/4R3/1kpR1N2/6P1/P1P2P1P/2K5 w - - 0 1"]

1.


I cover the solution for those who want to search: 


    1. a3! Kxa3 (1...Kc3 2. Ne2#) (1...Ka4 2.Nxe6 fxe6 3.Rxc4+ Kxa3 4.Ra5#) 2.Nxe6 Kb4 (2...fxe6 3.Rxc4 c5 4.Re3 Ka2 5. Ra4#) 3.Rc5 fxe6 4.Rdxc4 Ka3 5.Ra5#




The Greek Gift is another illustration of this rule because the Bd3, the Nf3 and the Qd1 coordinate to attack (the Bc1 and the Rh1 can also join the party)

[title "Greco 1792"]
[fen "r1bq1rk1/ppppbppp/2n1p3/3nP3/3P3P/3B1N2/PPP2PP1/RNBQK2R w KQ - 1 1"]

1. Bxh7!  Kxh7 2.Ng5 Kg8 3.Qh5 Bxg5 4.hxg5 f5 5.g6+-
Francesco di Castellvi vs Narciso Vinyoles

[Event "?"]
[Site "?"]
[Date "1475"]
[Round "?"]
[White "Don Francí de Castellví(mars aka love)"]
[Black "Narcís Vinyoles(venus aka glory)"]
[Result "white"]
[FEN "?"]
[Time "?"]
[TimeControl "?"]


1. e4 d5 2. exd5 Qd5 3. Nc3 Qd8 4. Bc4 Nf6 5. Nf3 Bg4 6. h3 Bf3 7. Qf3 e6 8. Qb7 Nbd7 9. Nb5 Rc8 10. Na7 Nb6 11. Nc8 Nc8 12. d4 Nd6 13. Bb5 Nb5 14. Qb5 Nd7 15. d5 exd5 16. Be3 Bd6 17. Rd1 Qf6 18. Rd5 Qg6 19. Bf4 Bf4 20. Qd7 Kf8 21. Qd8#


This is derived from the poem which you can read here. I found this while watching this youtube channel. 

At move 19 here I can see the three piece attack you were talking about. Here the bishop gets sacrificed for getting a checkmate. This the oldest usage of it that you'll find where the rules followed are the same as today.

Closest thing i could find was this sentence: 


   "3 Piece Rule Compiled by notyetagm  --*-- You need three pieces to
  checkmate the enemy king, one to sacrifice and two to deliver mate. Of
  course more is better. :-)"


Rule: one of a set of explicit or understood regulations or principles governing conduct or procedure within a particular area of activity. It should be absolute(a value or principle which is regarded as universally valid or which may be viewed without relation to other things)

But we know that it is possible to Checkmate your opponent using less than 3 pieces.
Example: For checkmate using single piece => Back rank checkmate or a smothered checkmate
For checkmate with 2 piece there is => classic queen rook checkmate or Diagonal checkmate,etc

Hence, it is not applicable everywhere and thus it isn't a rule (you shouldn't considered it either). 
"if you have three pieces or more in the attack you can sacrifice without thinking!" But this is a line that works a lot of time. So the question arises what is this? It's just a strategy or if you wanna be specific its a attack strategy or even a checkmate strategy in cases. 

You can even generalize this as "trade off when you have more number of attacking pieces that defending pieces". It actually establishes the basics of our chess today, that might not be fruitful in all the cases. As there is one and only one goal in chess , that is to defeat your opponent and obtain a victory.
So, Apply it all you want but make sure to win. Or at least its application should be fruitful to you and not your opponent.

As to my final answer to you, it isn't a rule it's a strategy and like other strategy's there is a very strong likely hood that someone played it before it got recognized by some name. As to oldest usage its  Mars vs Venus,1475 if you'll consider this. So, regarding who and when, it's something i feel that has evolved with time along with the modern chess.

Thank you!