I wouldn't be messing with connectors, splicing tools, crimping tools, buying bulk wire and cutting it to size etc.
Go to CTC or any other 'fast food' auto place (the same cables will cost you twice as much in a boat shop) and buy yourself TWO of these:
Once home, connect your 3 batteries this way:
You'll also need a 50 amp fuse OR breaker on your red wire between the trolling motor and the battery it is connected to. It's always better to connect that closer to the battery. A breaker is more expensive but is worry free. A fuse works just as well, and cheaper but you'll need to have a spare fuse or two on hand in case.
It's painless and shouldn't take you more than 5 minutes.
Good luck.