You cannot simply rename a .cxi file to .3ds and expect it to work. The internal structure must be rebuilt. To do this, you need the right software. Here are the most reliable "converters" used by the community.
Once the process is finished, check the file extension. You should now have a functional .3ds file. Common Issues and Troubleshooting Cxi To 3ds Converter
There is no single "magic button" app called CxiTo3ds.exe (most are command-line tools). Here are the three best methods. You cannot simply rename a