A better quality would make it much slower to compute.
If ever, you want to improve quality settings of level 3, you can compute a larger a image and reduce its size with an image tool. You can also set the properties com.eteks.sweethome3d.j3d.PhotoRenderer.lowQuality.antiAliasing.min to 1 and com.eteks.sweethome3d.j3d.PhotoRenderer.lowQuality.antiAliasing.max to 2, as suggested here.