Mr.SID MG3 images are not supported by ArcIMS Image Server 4.0, 4.0.1 or 9.0.
MG3 files are supported by ArcIMS 9.0 using the ArcMap server on Windows only
The needed DLLs provided by the vendor are still in a beta version and cannot be used by ESRI in developing a product.
Get help from ArcGIS experts
Download the Esri Support App