Okay, then yoiu just need a 'step down' power supply. A switched mode step down regulator is better than a linear regulator because it is so much more efficient, wasting much less energy as heat. There are plenty to choose from on eBay, which take upto 30V input and buck it down to 5V. They only cost pennies. EG http://www.ebay.co.uk/itm/LM2596-DC-DC- ... _777wt_932
(Just ebay 'LM2596' in your own country to find something suitable)
Your batteries ought to have a decent charging circuit built in, to manage the three lithium cells wired in series, which should include (hopefully) some form of cell balancing for when they inevitably go out of balance.
As long as they have an internal charge circuit and balancing then there is no problem charging them at the same time yoiu are drawing power - the problem isn't that the cells don't like it, but that the charger may end up being overloaded of it is designed to provide just enough current to charge depleted cells.....when you add a further load in parallel with the cells you may find the charger gets a bit warm !. Without knowing more about the load you intend to apply to the cells at 12V it's difficult to be more specific.
Hope that helps