The 2600 console was born ready to accept a controller with three buttons (i.e. the standard button over pin 6, plus two others via the "paddle" pins 5 & 9), yet it never materialized. If only Atari had introduced something similar to the Sega Genesis controller. The Booster Grip by CBS Electronics seems like another missed opportunity. CBS could have introduced a stand-alone controller with 3 buttons, rather than a band-aid fix for the CX40.
It seems the closest the 2600 has to a 3 button controller is the Sega Genesis controller. It has 3 buttons (A,B & C) and two of them (B & C) are already wired for compatibility with the 2600. Unfortunately, the A button is not wired correctly for communication with the 2600. So close, yet still so far. The only solution I can think of is to create a custom circuit board (based upon a simple design) which mimics the footprint of the board found within a Genesis controller. Then have a small run printed up and sell them to Atari fans who pop open Genesis controllers and swap boards, creating a 2600 controller with 3 buttons.