Russ,
I can give you some ideas about one of your questions. I'm not aware of any ESC that's designed specifically to handle three motors (doesn't mean they don't exist, though). There are some new versions now that do handle two motors, no idea if they would fit your requirements though. Probably the most common method of using ESCs with three motors is to use two ESCs. One handles the two outboard motors, the second handles the middle one. Depending on the boat, some make the center motor an 'on/off' thing and only control the two outboard motors with an esc. The third motor only being there for speed, if that makes sense. The rattings of the ESC used for two or more motors has to be large enough to handle the current requirements of both motors. It really doesn't make any difference how many motors are connected through the ESC as long as it's rattings aren't exceeded. That brings up another problem though. The motors have to be the 'same' (not particularly by brand, but in their current requirements. 'Unequal' motors mean one will get hotter than the other, and their speed won't be the same, which leads to directional control problems.).
- 'Doc

PS - If I remember correctly, the two motor ESC is made by 'Electronize'. (Don't count on my memory though!)