@Omaira
1 - It's a different game install, and yes the older TS3 for Mac version would need to be uninstalled first. Personally, I wouldn't bother trying the new version yet until we get a green light from EA that the incomplete install error problem has been solved. But it's not going to harm anything by trying if you already have another way to play.
2 - Yes, the Windows version has not changed and Bootcamp provides a different operating system for use. There's no connection between programs installed on your macOS partition and those on the Windows side.
3 - Catalina is not required for the new Mac version, the min requirement is High Sierra. But your hardware does need to be able to run the Metal API. Which exact Mac make and model do you have (from About This Mac under the Apple Menu)?
FAQ: https://answers.ea.com/t5/Technical-Issues-Mac/An-announcement-for-The-Sims-3-players-on-Mac/m-p/9636105#M8517
