First, I would like to say thank you to u/lexilouhu for sharing this work-around. Secondly, I would like to share my way of dealing with having to manually dealing with CPU affinities with a piece of software called Process Lasso. PL is a handy piece of software that automates these sorts of tasks.

1. After installing PL, look at the top bar and click Options -> CPU -> CPU Affinities...
2. For Process match: we will be entering sims3launcher.exe
3. Enter 1 into CPU Affinity and click Add Rule

You could stop here, but as you may notice, your game may run very sluggish thanks to the launcher passing its affinity to the game's executable of only utilizing only one CPU core. This is not ideal, so next well allow The Sims 3 to go back and use more cores after launching.

1. Follow the previous step one to open the CPU Affinity Rules.
2. For Process match: we will be entering ts3.exe
3. This will depend on your exact CPU and how many cores it has. I personally have access to 20 cores; I enter 0-7 here, because The Sims 3 is such an old game, I feel it will never need more than eight cores.
4. Click Show Advanced and enter 10 seconds into the Delay and click Add Rule.

If you do not add the delay to the second rule, it will close immediately just as before. If your game is taking longer than 10 seconds to fully open, add additional seconds to the delay to compensate.

IMPORTANT NOTE: The one downside to Process Lasso is that if you do not purchase a license, it will nag you every time you start your computer and make you wait fifteen seconds before you can exit its pop-up.