Welcome kravika.
If you want to get 3 inches and be done with it, you can meet your goal with just femurs. Tibia are cheaper, but it may be difficult to safely reach 3 inches -- you'll need to consult a doctor on this one, since how much you can lengthen laregely depends on your starting limb lengths.
You can answer vaguely, but what's your budget, age range, and region?