PLAY SoC FIRST.
Period, without a doubt, it must be done this way
The other two it doesn't matter which you do first. I say play CoP before CS just because it's a bit better game. Plus it's entirely new maps, whereas CS shares maps with SoC. This way when you get 'round to CS you don't feel like 'man, I just played these maps' like you would if you played it right after SoC. Mind you, what you DO in the maps is different, and they're not quite identical between games, but ... yeah.
That is the best way to do it. But not a big deal to mix up CS and CoP.
And it's not related to the time/storyline betwixt them ... in that regard they're like 99% unconnected with one another, practically all they have in common is that they're in the Zone.
What it is is that you'll really miss out on the 'magic' of SoC if you've already played either of the other two first. SoC *needs to be* a player's first introduction to the Zone. Once you've played all three, you'll understand why this is true