It was an insurance thing. Insurance companies were targeting V8 4 speed cars for big increases in 1972, so ma made a bunch of V8 3 speed manual cars to help customers skirt the rules a bit.
That may have been part of it, but the truth is a 3 speed is cheaper to manufacture. "You youngsters" have to remember than 4 speed gearboxes were a rare, expensive option at one time. EG there were VERY few 57 Chevs with factory 4 speeds
Well it's all relative. Chev wasn't any better. When I went into the US Navy in the spring of 68 I had TWELVE busted Chev 1st - rev sliders in the trunk of my parts car. The newer Mopar 3 speed was much improved, and stronger.........syncro into 1st gear.
At the time, I had gotten a 60 Falcoon. Those things had the tiny, 144 cube, "85 hp" six, and the 3 speeds in those were truely miniscule.
But / so keep in mind that when these 3 speeds were "in vogue" most cars did not HAVE all that much hp and torque.