Burning a lot oil and misfires in three cylinders is a motor that needs a rebuild. The cost is going to be labor rate. It all comes down to the total cost. If you can rebuild it yourself and willing to get dirty, then go for a rebuild. If not, buying a used longblock will probably be a better option and make sure you get warranty. Other questions to ask yourself that can help you decide are; how long are you going to keep the car in your possession? Are you planning to modify your car with after market parts? If your not planing to keep it for long then just by a used longblock. If your planning to modify the motor then now is the time to do it. Good luck.