Basically what they said was that if you want to haggle on price then the 3% is not applicable. But at the end of the day they want to move cars so there is some leeway. Just not as much as you would off normal finance. I managed to knock a little over $100 off my monthly repayment from the first figure they gave me. So over 48 months that is still almost $5k.
I don't think they will go for 8 - 10% because they would have to lose money on 3% interest.
The other thing is they would try to flog you a 320d or 320i - they are very slow moving so who knows what they might be willing to do?
Good luck with your negotiation.