The dark side appears because you have what is basically a cube, and the light is coming from a direction where it's casting a shadow on the other side. Move around your lights and see. There are a few settings in the .m3 exporter that you could play around with to make it not be affected by lighting, if that's the effect you wanted. The other suggestion I would make is to give it some bevelled edges so light can catch around the corners more smoothly.
If there is an issue beyond this, show us a comparison between your box and similar working object (building, doodad) that has the look you want.
The dark side appears because you have what is basically a cube, and the light is coming from a direction where it's casting a shadow on the other side. Move around your lights and see. There are a few settings in the .m3 exporter that you could play around with to make it not be affected by lighting, if that's the effect you wanted. The other suggestion I would make is to give it some bevelled edges so light can catch around the corners more smoothly.
If there is an issue beyond this, show us a comparison between your box and similar working object (building, doodad) that has the look you want.