If all three are avid fishermen, then you can expect quick adaptation to the circumstances. Productivity will be higher once you find what's working, and you'll find what's working quicker if you keep everyone trying different stuff. Be careful while casting.

Anticipate more time maneuvering the canoe to release snags.

Canoeing with three paddles can get wobbly during hard pulls. You'll have smooth and fast sailing if stern paddler establishes a moderate, even pace and the other two keep time with him. Put stern and middle on opposite sides, then bow chooses side based on wind, current and relative strength of two front paddlers. You'll notice a BIG difference when the middle paddler stops to check bearings on map.

We like to rotate seat positions after every portage, although you should be careful of creating awkward distribution of weight. All three of us weigh about the same.