I would be VERY concerned about the 3 mile commute. I would consider that the absolute most severe service you can put on any engine - gasoline or diesel. But it will be especially hard on the diesel because it takes so long to get up to temp. I drive my 300D 18 miles one way to work and back and the tranny fluid barely gets hot (55 mph two lane). I only drive the E320 on weekends and never take it out of the garage for less than 30 miles.
If I lived 3 miles from work I believe I would be taking the bicycle every day.

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Jim