Conrad, I hope you are saying that smoke backdrafts down the chimney of the one with the fire and not the others. If you are certain that the chimneys free from obstruction and correctly sized, about the only other cause would be depressurization of your house (not uncommon with forced air heating). Test this theory by opening windows or outside doors in the room where the fireplace is burning. If it starts to draft normally you have the cause. Why don't you try this and report the results; then I can outline some possible causes of the depressurization.