I also thought 3 speed manuals were not a good choice for high performance cars back in the day, but I learned differently.
In 1970 I purchased brand new a Duster 340, 727, 3.55 Sure Grip.
A neighborhood acquaintance also bought a 1970 Duster 340 with the factory standard 3 speed manual tranny on the floor, but he also added the 3.91 Sure Grip rear.
That car was very tough to beat off the line and was very strong! I saw in disbelief I might add, that car beat even 4 speed 340 abodies! The gearing that was matched between the first gear in the tranny and the 3.91 SG was unreal!