After some changes...
> #version 3.7;
Mendatory:
global_settings{assumed_gamma 1}

>
> #include "textures.inc"
>
> //difference
union
> {
>     box{<-2,1,-2>, < 2,4, 2> }
Remove the useless union.
>    // union // This is not needed at all
>    // {
New colouring.
>        box{< 0,0,-3>, < 3,3, 3> pigment{rgbf<1,0.1,0.1,1>}}
>        box{< 0,2,-3>, < 3,5, 3> pigment{rgbf<0.1,0.1,1,1>}}
>    // }
>     pigment{rgb<0.1,1,0.1>}
> }
>
> light_source { <5,3,0> rgb 1}
This light is BLACK!
You must add some colour.
>
> camera
> {
Pull the camera back so we can see the whole thing
>    location <5,3,-9>
>    look_at <0,3,0>
>    sky <0,1,0>
> }
>
> Rendering the above scene creates a box with some kind of hole in the middle.
> The boxes in the union must be like a big box {< 0,0,-3>, < 3,5, 3>} but the
> behavior is different.
>
>

There is a construction problem as well as coincident surfaces that 
cause some additional problems.

Changing the difference to an union.
Giving distinct colour to each component...
Pull back the camera to -9*z
Adding a colour value to the light_source
and we get:

A smaller green box, a large red and a large blue box.
Most of the green box is inside the others.
The red and blue overlap.
The red and blue boxes have coincident surfaces. This can throw 
insideness tests off.

The problem is NOT version dependent, version 3.6 or 3.5 should render 
similarly.


Alain

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