In a basic way, you can think of batteries as energy reservoirs. You had a boat with a TM that used two buckets of power to move your boat during a day of fishing, where each bucket of energy was provided by a Group 27 battery.
Now you have the same workload (your boat + contents) that you want to move just like you did before. However, now you have a 36V TM that happens to have THREE buckets of energy available. Your new TM actually draws energy from those batteries at a lower amperage level than the 24V motor did. If you were good with two Group 27s previously, you'll find that you don't drain three Group 27s as deeply as you did when you ran with two.
When you eventually need to replace them, if you've been happy with the run time, then stay with 27 and save a few bucks/pounds, or if you want even more energy, bump up. Generally the addition of a 3rd battery is a big plus that should make for happy fishing days.