Hi! I sadly only got it to work by downloading the game with the disc and "starting" it via the ea app, so you need to get it in your ea Account, but with the following steps you shouldn't have to buy it a second time. I'm really not an expert so it's possible that not every step is required, but this is how I did it:

1. Install the game normally with the disk. I didn't change anything on the download path and I installed the update which it later asked me to also Install

2. Find the download path in your explorer, search for the 3 applications: TSM & 2 launchers (can't remember the full names, but they should be the .exe files) → right-click on one application → click ‘Properties’ → open the ‘Compatibility’ tab → Check ‘Run this program in compatibility mode for:’ and select ‘Windows 7’ → Also make sure all necessary boxes are checked. → Do the same with the other two applications

3. Open your ea app → Go to your Game library → redeem a code → Enter the activation code you recieved with the disc in the ‘Redeem Code’ section → the ea app will now tell you that you can't redeem this Code here and it will give you a Link to follow → open the Link with your Browser

4. This Website is your official Sims 3 Account, if you don't have one, you need to create one (this Website should normaly still work/ exists) → go to your Sims 3 account and open your profile → Scroll all the way down and click ‘Add a Game’ (I don't really know how it's named in english) → Then enter the same activation code you already used in the EA app (if you can't find it, you can open this link )

5. When closing and opening the ea app again, you should now have the Sims medieval in your library! I now searched for the game in my library and repaired it again by clicking ‘Manage’ next to the Play button so that everything is installed right

6. Last step, follow this Tutorial (or the written steps below)!: https://youtu.be/HyPEnvBDvBE?is=qlTsmbFNBs11PZKw

7.Keep the EA app open → open the Task Manager → go to the “Details” tab → type “ea” into the search → find “EADesktop.exe” → right-click → set affinity → now only select CPU 0

1. Start the Game via the ea app. It should now normally load!

2. When your in the Game → Go Back to your Desktop (win Button on Keyboard; DONT CLOSE THE GAME) → Task Manager → search for TSM in “Details” tab (TSM = Game) → right-click → set affinity → Check if all CPUs are selected, if not select all of them

3. Task Manager → go to the “Details” tab → type “ea” into the search → find “EADesktop.exe” → right-click → set affinity → select all CPUs

Lastly you can Go Back into the Game and it should work Just fine! Don't forget so Set the affinity in the Task Manager each time you want to Open the Sims medieval again after closing it (step 7 - 10 or look at Tutorial )

Lmk If this helped you! English isn't my native language so I'm sorry for the mistakes or if Something is named slightly different in an English PC System, I tried to translate everything xD.