The two trolling motor batteries should remain trolling motor ONLY batteries.
IF the current starting battery has the capacity to handle the rest of the loads, just connect your added power circuit to that battery.
Since you are adding more sofisticated computers to your electrical system, you may need a larger battery with more reserve capacity.
For a common power connection for your electronics, add a power buss at the console using 10 guage wire with a 20A circuit breaker at the battery on the postitive wire (RED). Then use a properly sized fuse to each device's power cord at the buss.