you might face some issues with the file format so these tools might help you also you can search in YouTube and google.
Note: if you can run the USBLoaderGX app on the Wii, means that you can play the games in the USB. I suggest first installing the USBLoaderGX in the Wii No.3. Also There are some tutorials how to do it in YouTube.
1- File with format .iso.dec convert to .iso
Download the link down and extract the files into a folder, to extract a rom drag the rom onto the .exe the command prompt should popup and extract the ISO from the .dec file.
2- File format .iso you want to convert it to wbfs
best to use this tool to convert its very easy to use just need to read the options.
3- USBLoaderGX is a GUI for Waninkoko’s USB Loader, based on libwiigui.
It allows listing and launching Wii games, Gamecube games and homebrew on Wii and WiiU vWii mode.
Its interface, based on the official theme from Nintendo Wii, is easy to use and perfect for kids and all the family. A lot of options are available and directly editable from the loader’s interface.
1. Extract latest revision to your /apps/ folder
2. If you want a channel on your console, and/or want to “return to” the loader when exiting a game:
Download the forwarder for your console:
For me was really easy because in my external usb drive I already had installed the USBLoaderGX app. When this is installed, it creates a WBFS folder in the drive, and you inly need to do is to copy and paste the game .wbfs file into it.
The key is to copy the .wbfs file in this folder including the game ID in the brakets [ ] so the Wii can recognize it without issues
for other formats, i.e., .ISO, .DEC or .rar or .zip, you need first to decode or unzip the files to get the .wbfs file
I suggest to install in the PC the Wiibackupmanager app. There some videos in youtube how to do it and use it.