If you were sold a new vehicle, and it turns out not to be the case, then you can reject the vehicle outright and the selling dealer would be due to repay you the full price of the vehicle. Misrepresentation is the word that is relevant here. They sold you something that wasn't as described. Happy to discuss further. I hope that helps. Please leave a positive rating so that I am credited for my time.
Thanks for your clarification. As you'll know many vehicles are stored before sale and the date of sale is normally the date that the warranty starts. However if Mercedes Benz are not recognising that, and I think you should query it anyway, the. Your remedy is against the seller company. It may be that they will sort this for you with Mercedes but if they stick to this then you have to claim misrepresentation as previously advised.
Unless the dealer can get Mercedes to reset the warranty, what has happened is that they have sold you a three year old vehicle when they said they were selling you a new vehicle. That is misrepresentation irrespective of when the vehicle was registered. Your remedy is against the dealer. Their way of dealing with this should be to give you their own warranty for the period that you were told you had a warranty from the manufacturer. It's either that or you reject the vehicle and claim damages.
If I was you I would follow my original answer on 8 August and reject the vehicle.