If the car was bought from a dealer and they gave you the three month warranty, it's down to them to sort it.

There are many things that could cause a mis fire, and without a proper diagnosis first, it'd be pure guesswork to say what's causing it. It could be (depending on what engine it has), plug leads or coil pack, a wiring fault, faulty ECU, head gasket failiure, burnt out valves etc, etc.

Driving a mis firing petrol engined car can and will cause damage to the catalytic converter in quite a short time as un burnt fuel destroys the innards of the catalyst. You're doing the right thing by not driving the car, but you need to insist the seller gets the car recovered to them for diagnosis and repair of the fault.

Good luck with it, and stand by your guns when telling the seller you want it fixed at no cost to you under the warranty.