You don't want to introduce objectionable noise as you likely would by using a switcher.

Since you don't have line regulation issues, the source being a battery, and since current is small, you might want to consider a "rail splitter" such as the one from TI that I ran across while unpacking just a few days ago. It's still lying on one of the worksurfaces in my lab. IIRC, it allows you to produce a reasonably low-impedance ground potential between the rails of a single DC supply and, set the offset between the two rails. that will give you +12 and 0 volts converted to +7 and -5. Since all you want is 5V0, 3V3, and that negative supply, perhaps it's practical to use a different approach, with op-amp's and thereby minimize the power dissipation. Keep in mind that most linear regulators, quiet as they are, regulate by generating heat. That heat can work against you, so if, for example you can use 6 volts rather than 5, you can see the advantage. If not, well, there are ways of wasting potential, starting with, perhaps, routing the input supply through a darlington. The heat lost there won't exceed the heat lost otherwise, unless you opt for a noisy-but-efficient switcher.

Kai's suggestion of a "simple switcher" will do the trick, certainly.

RE