When you open RTSS, you can create a profile for TS3.exe (Origin install) or TS3W.exe (with the W, for disc and Steam installs). You should then have the option to set your fps to whatever number you want. It should match your monitor's refresh rate, which is likely 60 Hz (you can find it by running another dxdiag—it should be listed under Display Devices, as "Native Mode," about twenty lines down from the header).
https://help.ea.com/en-us/help/pc/how-to-gather-dxdiag-information/
You should also see an option in RTSS to pin it to startup. Then it will be running at all times, and you won't have to remember to launch it before playing. It takes up almost no resources and doesn't require your attention once it's set up.
igazor posted a screenshot of his settings a while back, in case it helps:
https://forums.thesims.com/en_US/discussion/comment/16227429#Comment_16227429
Don't worry about TS3's built-in fps setting. It doesn't do anything useful. I just leave it on default.