If they didn't drain your torque converter, then they only drained approx. half of the Dexron-III your system holds.

Imagine a quick-lube place telling you that they started an oil change on your 6 qt. engine and stopped when 3 qts had been drained out. Same thing as far as I'm concerned.

Find another repair facility or do it yourself.
Tranny fluid changes are not hard to do on most Benz cars.