In all my ML's, my FSS service came between around 12,000 miles and 16,000 miles.
http://www.4x4abc.com/ML320/ML_FSS.html
3000 Miles is not normal for an average city/highway used ML. Your dealer should understand that. I am sure you are not in low range and racing through the sahara sands/dunes. Have you checked the condition or your oil..? At 3000 miles your synthetic oil, and filter, should still look good compared to oil at 16,000 miles.
Some possible other remote possibilities are:
They did not reset the FSS and only topped the oil instead of performing the A service oil change therefore making the FSS sensor extend the miles before the service warning. If you just top the oil, your FSS sensor will think the oil is in better condition and will extend the warning for later. Maybe 16,000 is the hard limit for maximum FSS miles.
Bad oil sensor..?
Somebody added dino oil instead of synthetic oil and therefore the shorter dino oil life was detected by the sensor?