Smoke chamber in Fireplace...install Mansfield?

[geka]

We have decided to get a Mansfield to replace our VC Defiant. Now to the issue of installation. We had our chimney lined with permaflu 24 years ago. They sent down three 6 inch flues to about the floor of the second story. They left the smoke chamber open. The Fireplace is on the first floor. Right now we have some galvanized steel with an 8" cutout that we have our defiant pipe through. It is at the bottom of the smoke chamber so the venting of the stove goes right into the smoke chamber and then through all three 6 inch flues. From cleaning these flues out we know there is one favored flue although there is evidence that there is some draw from the other two. Do you think we can just pipe the Mansfield pipe into the smoke chamber or should we hook it up directly to the one flue? It is going to be a huge job to do the later. Should we run flexible pipe through the entire flue that we use if we use just one? I am sure I will have more questions but will start with these. Thanks ahead of time.