Need guidance on selecting proper Gates belt drive components for my belt-ready frame
The Gates CDX system is supposed to be a bit better than the CDN, but they share the same belt and sprocket design. I believe the rear sprockets are interchangeable. CDC is a different design without the center guide.

You want the 3-lobe sprocket to fit on a Nexus SG-3C41 hub. Videos of how to install these are readily findable on the Web.

Without knowing what crank you have it's hard to give specific guidance. You need to know the bolt pattern and bolt center diameter (BCD) of your crank to find the right front sprocket.  

The trick with Gates belt drives is that they are pretty sensitive to alignment of the front and rear sprockets. You need to know the chainline of the hub and sprocket and make the front sprocket on the crank match that. )Poor alignment may be why your current setup keeps throwing the belt.) If you have a square taper bottom bracket and crank you can adjust chainline by getting a different bottom bracket with a different axle length.