Thanks for responses guys,
i have bought the vehicle and verified its sales invoice and sales certificate issued by Paksuzuki on Dec-07. They have verified the serial number and sales documents and told me its 4 cylinder car. As they stopped booking 3 cylinder car in april 07 & no 3 cylinder vehicle has been manufactured after july 07. Also the price on which this vehicle has been purchased is of 4 cylinder. So this is mistake done by excise department and you can modify this by taking vehicle to lahore motor registration authority.
Thanks