I am terrible at explaining stuff but here goes. I am going to explain it like you have no current save except the one you downloaded. The RAR file was named d3save correct?
1. Extract that file to your desktop or wherever.
2. Open Horizon. At the top you will see a button that says Tools. Click that and then click Package Manager.
3. A box will open that says Open,Rehash&Resign and some other things. You will notice 3 boxes in the middle that say Profile ID, Console ID, and Device ID
4. You need to replace the values in those boxes with your own. You can get these by opening a save from another game with the Horizon Package Manager and copying them.
5. After replacing the values in the 3 boxes with your own click Save,Rehash,&Resign.
6. Right click on your games folder in the Horizon device manager and you should see an option to Inject file.
7. Inject the d3save you just modified. Start the game and see if it works
It should work. I did not mess with the contents tab at all until I used Idlehands editor. Seems like you did when you got the value.
Sorry if this was not helpful. Good luck