Even for new cars, most manufacturers say normal oil consumption is almost a liter/1000km.
BTW, Just out of curiosity, how did you calculate that you're 3.5 liters low? Seems very unlikely that the engine wouldn't have destroyed itself with more than half of the oil missing.
Also, if you have an OBC, you should get a warning on the dash upon shut down when it starts getting low (<1 liter missing); if not, then do what you should be in the first place: check it once a week, or at least once a month at the very least. I can't imagine driving for that long and not once checking the oil level.