A used MB should come with the outstanding balance of the manufacturers' warranty. Normally three years. So a three month old car would still have 2 years and 9 months left.
If bought as an Approved Used Car (AUC) it would normally also come with a 1 year Approved Used warranty from the date of purchase. But with a 3 month car the manufacturer's warranty still be in force. The AUC warranty would only really be of benefit of you were buying a AUC car over 2 years old in which case it would extend past the end of the manufacturer's warranty.
If not bought as an Approved Used Car then the 2 year warranty is presumably someting different. Dealer specific? In which case it will have its own terms and conditions. It may also be possible under some circumstances that the manufacturer's warranty doesn't cover the car .... that would be very unusual if you have bought the car in the UK and it is UK stock.
So .... you need to check and read any paperwork you have ..... or ask the supplying dealer.
Don't the AMG models come with 2 years approved warranty?
Of course, the warranty for any specific vehicle was no doubt discussed and agreed with the selling garage and confirmed on the paper work that was signed when purchasing the vehicle. No body would buy such a car without asking and understanding the warranty....