Mike, my suggestion is to locate a local dealer to see if they have an identical Mad River canoe that you can measure for the seat placement distance and height for each seat. I’d also take along a digital camera for reference when you get back to your shop.
Stability on the water- I’d be leery of adding the third seat. But if you d, don’t mount it high where a yoke would be placed because the canoe will be even more unstable.
Consider using a Crazy-creek seat on the floor of the canoe for the duffer (third person).
You might also check the Mad River web site to see if the canoe is still in stock. Their info @ email may answer your questions. But the surest way is to do it yourself if you have access to another Freedom.