I have A Sachs Pentasport (P5) that was on a bike thrown out in Germany in 2000 or 2001. It is the two cable model with coaster brake. I much prefer the Sachs/SRAM Spectro S7 with Coaster brake that I have been using for commuting and shopping for over a decade. It feels much smother and does have the extra gears with no disadvantage.
I wouldn't pay much attention to theoretical "efficiency" charts, the proof is in the riding in an all up bicycle.
I had very good luck in the past with S-A S5 two cable hubs, the first version of S-A five speed hubs. Used one for almost 25 years without a problem. You might want to look around and see it any are available in your area. If you are handy, the S-5 internals will fit perfectly in any S-A AW three speed shell.
Pictured is my dumpster Sachs (SRAM) 5 speed dumpster find.