The straight 6cyl-SOHC motors used in many of the 300Es and 300SE/300SELs revv high. I was used to the lower revving Honda/Toyota cars B4 I acquired my MB 4 yrs. ago. It's also possible that you only feel 3 and not 4 shifts. Many also start out in 2nd gear by design. You get 1st gear by stomping the accelerator pedal.
3000rpm at 60mph in your car in quite normal. You're likely seeing 3400-3500rpm at 70mph.
1991 300-SEL - Model 126
M103 - SOHC
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