Depends what the third battery will be.
If its the same type & capacity as your second then easy just run them in parallel with big cable and circuit breakers either end.
How old is the 2nd battery?
If its getting on a bit it would be best to replace it as a matching pair with the third and run them both as a bank off the Ctek, if its big enough (most likely will be - 250s will comfortably look after at least 200AH and up to 300AH by their specs).
If you need to run the third as a different capacity or type then you will most likely need to run a 2nd DC-DC and opinions will be divided as to whether you should run this also off the main or 2nd battery.
My opinion - run it off the main it has the best access to the charge current available from the alternator. No point charging one battery that is then charging another, why not charge both at the same time.
Might be cheaper in the long run to replace your 2nd battery at the same time as doing your 3rd, depends if your Ctek is up to the task. At least you end up with 2 new batteries that you can run as a pair and no need for more expensive DC-DC chargers.
The other advantage if you run the two as a bank is you can just utilise all your existing cabling for power outlets, no need to change anything there. It all stays the same you just have double the capacity.
If the third is run as an isolated battery you will need to work out how you're going to get the power out of it.