If you can wait a week, I can probably get you measurements from a Reflection that was set up by Dagger with three seats. A woman in my canoe club has one. Dagger may not have perfected the set up, but if you follow their set up, you probably won't go hideously wrong.
Personally, I think I'd add the thwart behind the center seat so it would be less in the way. However, I am not a canoe designer.
Recreational Kayak Paddle
Kayak Motor Kit
Free Standing Boat Racks
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