It depends on the content of the poster
Generally 8-bit is fine unless working on fine-art reproductions or other images where the tonal range is wide. It also depends on the method of reproduction as well. There's no point in using 16-bit if the poster is going to be photocopied... or even digitally printed, let alone off-set.
Depending on the version, many PS functions are not fully available for 16-bit images.
If it concerns you, then work in 16-bit and save an 8-bit version of the final artwork. This may help avoid banding in gradients and other artifacts like posterisation.
If doing extensive Photoshopping on non-natural images, I very rarely work in CMYK exclusively until towards the end of the process because many functions aren't available in CMYK mode. I prefer to use the CMYK preview (View>Proof Colours = Apple-Y) to get an idea of how things are going to look when working in RGB for artificial subject matter and intended for press.
600ppi is a little high unless you plan laying out type from within Photoshop. Bear in mind that an A3 poster will not be viewed as closely as a publication or other smaller piece of work.