Jeez.. bunch of amateurs, no serious answer.
If you want really good quality (300dpi):
For paper 3x3m resolution should be 36000x36000px.
If you don't need so high quality, and you will look on that image from more than 2m, 76dpi will be enough. And it give you resolution 8640x8640px.
About colors, start working with RGB and after all convert it for CMYK. Why? because on CMYK not all filters are available.
And 8bits.
It's not about your free hd space. Increase the amount of Virtual Memory.
Aspect ratio? Seriously? You want image 3x3m and you are asking about aspect ratio?
Aspect ratio is the relationship of an image's width to height. So in your case 1:1.