One obvious problem is that most files are viewed on a monitor in RGB and then printed in CMYK. There isn’t space here to argue the merits of individual color spaces, other than to say that most designers should work in Adobe RGB (98), which has quite a wide color gamut.
There are several CMYK color spaces. The main advantage of A3 is that it is used for offset litho printing and so it will be easier to match jobs to other print processes, such as ensuring that posters tie in with packaging. But most wide format devices can produce a wider color gamut than offset presses and you may not want to limit this.
The standard ICC concept involves using a color matching module or CMM to convert RGB colors firstly to an independent LAB color space and from there to CMYK. Both Windows operating systems have their own built-in CMM, as does the Adobe Creative Suite and most color management software.