If the game you're working on uses CatSystem2, you can go here and get the official toolkit for the engine.
Inside the zip file, look inside "tools" for a program called WGC and start it up.
You should see an empty window. Drag and drop any image files you want to convert into it and you will see the file paths appear on the list.
On the top-menu bar, click コンバート!(C)
Click on the first item: 全てコンバート!(A)
The newly converted hg3 files will appear in the same folders as the original image.