I wish I could give you more technically relevant advice (I'm completely unfamiliar with CC3) but I can give you some general ideas.
Your arctic areas seem too regular, in terms of that zig-zagging, and they seem really big in comparison to the other continents. I use GIMP, so I'm of little help for the actual method, but I tend to use noise filters and blurs to make my terrain mesh well, which is a problem you seem to be encountering.
The problem with your rivers might be resolved if you placed a lighter blue under your continent layer(s). Generally a blue closer to sky blue/cyan works best, and can also function as an excellent blue to surround your continents with a bit, in order to convey the shallow water present at the shore.
As for your deserts, I would use a more brown/tan type color (though be wary of making them too much like your mountains in terms of coloration). If you were using GIMP I'd tell you to bump map your desert layer with some rendered Plasma, in order to get a granular texture that should work very well.