The first thing to do is find out how much current you really need on each of the rails. When I see 3 micro-controllers, I think more like 300mA not 1.2 amps.
In any case, that switcher looks like it is up to the task for your application. At 12vin and 5vo @ 1 amp, it is 92ish % efficient and will need to dissipate about 0.5 watts. At 12vin and 3.3vo @ 1.2 amps, it is about 87% efficient and will need to dissipate about 0.5 watts.
aishiqi
You don't want to use a 12v to 5v linear regulator here as it'll need to dissipate about 7 watts. It'd need a large heat sink and maybe even a fan.