I would like to add 3 FBX imports and make layer 1 to render only in the red channel of RGB image. I would like layer 2 to go into the green channel and layer 3 go into the blue channel.
Like a triangle object to render in red channel.
Like a circle object to render in green channel.
Like a square object to render in blue channel.
Is that possible?
It’s for sure possible 
there’s quite a few different ways - the easiest would probably be to export the RGB layers as separate images (you can do this by changing the render in the render preview and then Save Render as Image), or you could plug each of your objects into an emission node set to pure red, green, or blue, with no shading, or you could plug each of your objects into a Diffuse BSDF, plug that into a Shader to RGB, and plug that into a color ramp from black to pure red/green/blue if you want shading
OR, you could set a compositor separate RGB node with the rendered layers as a feed and connect each one of the RGB channels to a file output node. Blender has many ways to do things. Choose your flavor. 
If the complete objects are wanted to be rendered then this should be different scenes ? (did do that for myself…) And the result joined accordingly to R, G and B…
