I checked the source code of GraphicsMagick and they also only convert the colors when there is a source profile. Your 3convert.jpg image is still CMYK and 3convertRGB.jpg only had its colorspace changed and was not converted. It looks too blue on my monitor. With the following code I come really close to your CMYK source image.
The colors will never be the same but the colors of 00000003.rgb.jpg are closer too 00000003.jpg then your 3convertRGB.jpg image. When you look at the bottom 'Sugerdale' logo the color is too red compared with the original image. And the meat on the right is too purple in the centre.
using (MagickImage image = new MagickImage("00000003.jpg")) { image.AddProfile(ColorProfile.CoatedFOGRA39); image.AddProfile(ColorProfile.SRGB); image.Write("00000003.rgb.jpg"); }