Using a 48V Charger for a 12V load
No!  Your diode scheme won't work.  It's not clear how exactly you propose to connect the diodes, but either you will short one or more batteries or you will end up with the full 48V minus two diode drops on the output.

As Leon said, what you need is a 48V to 12V converter.  The most likely form of this is a switching power supply called a "buck converter".  You can look up that term for details on how exactly they work.  At your current (almost 300A at 12V out), this will not be a trivial design and it will dissipate significant heat no matter what you do.  Unless you're skilled in switching power supply design (and if you were, you wouldn't be here asking), you're better off buying something off the shelf if you can find it.  This is not a beginner project.  Do something else first if you want to learn.  At this power level and current, you'll need every possible trick including multiple phases and synchronous rectification.

Consider the cooling requirements.  Even if you can make the converter 95% efficient (that would be impressive), it will still dissipate 150 W of heat when operating at 3 kW.  That is not a trivial amount of heat to get rid of, and will require careful attention to cooling.  In other words, this isn't just a expert level electronic design problem, but will also require more than casual mechanical engineering for the thermal management.