3-speed

I have searched the internet like a mad man and cannot even find general information on the 3-speed manual transmission 1972 340 Duster that I own.

What's the purpose of having a 3 and 4 speed of the same year and model.

Are 3 speeds rare? They must be rare.

Can someone explain to me the differences besides the obvious ones? There has to be a reason for this. What are the advantages of having one over the other? Let me say this, with my 3-speed, once I'm in the 3rd and final gear the car feels like it needs go up yet another gear.

To anyone who can fill me in on this, I appreciate it.