@AndyZhang2896, it's been 5 years since I played TS3, so some of the details I might not remember exactly. 😳 However, if I remember correctly , the expansion worlds in TS3 were separate. ☹️ You could not travel to them for a visit like you can in TS4. An exception were the vacation worlds, but you would stay there at least a few days. Vacation worlds had loading screens when you traveled to and from them.
But the worlds your Sims lived in were separate. I think there was a way to move to another world, but I could be wrong about that 🤔. Also you wouldn't live in one world and go out to eat in another world , for example. So, in TS4 if you were playing the Goth household and you wanted to visit your friends The Pancakes, because they are on opposite sides of that "world" (in this case Willow Creek) you travel by loading screen, but in TS3 you would just walk, bike, or drive there (or ride a horse if you had the Pets EP). And if the roads were winding it could take a long time, especially if they walked or rode a bike. Some of the worlds were big, which didn't help matters. And it would really take a long time if you had all the packs installed and/or didn't have the best computer. 😞
So, for TS3, you couldn't really travel between expansion worlds (unless on vacation). 🤨