What you will end up with will be three separate battery chargers in one box.
To get isolation, you need a transformer. As, I assume, you will be powering this from mains electricity, you will need three separate transformers to get isolation as well as the three voltages that you need.
An idea how much current you are going to want the chargers to be capable of? I guess this is for lead-acid batteries.
A mass-produced product to that specification would probably use a custom-built transformer with multiple output windings. However I would not recommend that for a project, because building a custom transformer is a lot of work if you want it to be anywhere near as efficient and neat as a standard one.