When you hook up 2 12-volt batteries to create a 24-volt battery, it is
best if the two batteries are the same brand, size, and age.

This means this is not a good ideas to adding a new 12-volt deep cycle
battery in serie with the existing 12-volt deep cycle battery to make
up a 24-volt. I understand this now. Thanks.

Get two new 12-volt deep cycle batteries for the job ...

I don't want to see four batteries in the battery compartment. This
would take too much room and weight, right. Sound like I should get a
24-volt battery as the third battery just for the trolling motor.

Otherwise, lean toward actual "deep cycle" rather than combo
duty batteries. You can always use a deep cycle to start an engine (if
it is rated for enough CCA), ...

Yes, this is good to know this.

If you leave the dock with a fully charged trolling motor battery, it's
unlikely that you will need to recharge it during a few hours of
fishing. Unless you're going to be away from shorepower for days at a
time, why even worry about recharging the trolling battery from the
alternator? Get a decent marine batter charger dedicated to the
trolling battery and recharge at the dock or back home in the garage.
Trying to charge 12-volt and 24-volt batteries at the same time from a
single alternator would be pretty tricky. I always overdo everything,
but if that were my problem I'd add a second, 24-volt alternator and
keep the two systems separate.

Good. This means I don't need to worry about charging the 24-volt
battery using the alternator. I don't need to do this, and I would
have a lot of trouble doing this even if I tried. Thanks for helping
me to make the decision easily, and that will be adding a 24-volt deep
cycle marine battery (that matches or exceeds the cranking power (CCA
rating) of the other existing batteries) as the third battery just for
the trolling motor.

Thanks again!

Jay Chan