I was at first shocked that a Mondeo not yet 5 years old would have such little value, but a quick search on Autotrader shows 783 Mondeos nationwide upto 5 years old, valued between �3K and �4K so it's probably in the right ball-park, if a little low.
How come you still owe �6K on it? Did you pay through the nose or what? How long have you had it?
Lastly, how do YOU know it's only worth �3K, just because that is what you were offered? - still sounds too cheap to me (depending on the mileage and condition of course). I would check for similar model, age, & mileage on Autotrader for a good clue as to it's actual value, not just go by what this (apparent) con-man has offered.
As someone else has said, in another year it won't be worth nothing - as it ages then the depreciation slows down. In another year it will probably be worth �2-2.2K.