That's exactly what it's supposed to do.

At road speed, these things have a lot more than 3 bar oil pressure. The designers simply assumed (For some unpleasant reason) that the only "useful" information was the bottom end of the gauge. in other words, finding a close focus on the "low" end of the spectrum was all that mattered, because there's not really any such thing as oil pressure being too high, so the only MEANINGFUL message the gauge could provide is if it's dangerously low.

As such, the 3 is just an artificial limit of the gauge; it can actually climb significantly higher than that. Every cold start should see it peg at idle because the oil is cooler and thicker; this is a phenomenon in ALL engines. It's just more visible on the Benz because the gauge is so narrow-ranged at the low end. Ambient temperature and engine temperature as well as oil viscosity (at the immediate moment; not just the number on the bottle) all play a part in dictating whether the idle pressure will drop from 3 or not. Sometimes it does, sometimes not. USUALLY on a warm engine, it's more like 1.0-2.0 bar. However, I wouldn't assume it's a PROBLEM for the thing to stay at 3 even at idle.

Probably perfectly normal, in other words.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL