Hi
There are two different versions of Apple Watch (not iwatch) Series 3.
Apple Watch Series 3 (GPS) does not have a built-in cellular capability. Series 3 (GPS + Cellular) models support a built-in cellular feature, subject to having a compatible service plan from a supported carrier:
Apple Watch Series 3 (GPS)
Apple Watch Series 3 (GPS) models do not have a built-in cellular capability.
Any calls or data-related services are routed either via the paired iPhone or via a direct connection between the watch and a compatible Wi-Fi network (when the watch cannot connect to the paired iPhone via Bluetooth).
A separate cellular plan for the Apple Watch is not required.
Apple Watch Series 3 (GPS) can be used in any country in conjunction with an iPhone 5s or newer model of iPhone (running the latest version of iOS) and a service plan for the iPhone from any carrier.
Apple Watch Series 3 (GPS + Cellular)
In respect of data usage, Apple Watch Series 3 (GPS + Cellular) also always first tries to use the Bluetooth connection to your iPhone (which does not require a separate plan for the watch) or, in the absence of that, any available connection to a compatible Wi-Fi network.
If neither of those is available, then Apple Watch Series 3 (GPS + Cellular) has a built-in cellular capability that, under certain circumstances, can enable certain features, which are not available on GPS-only models, to remain available when the watch is used without the iPhone nearby.
Apple Watch Series 3 (GPS + Cellular) is not locked to any specific carrier and has a built-in eSIM that copies information from the iPhone's SIM card.
The built-in cellular features require a carrier plan for your iPhone in your country of use that specifically includes support for Apple Watch Series 3 (GPS + Cellular), including the carrier activating the device on the plan.
If buying Apple Watch Series 3 (GPS + Cellular) in another country, then, in order to use the built-in cellular feature, you would need to buy a model that supports the relevant LTE and UMTS bands for your intended country of use and your supported carrier there.
There are three different regional versions of Apple Watch Series 3 (GPS + Cellular), each supporting different LTE and UMTS bands, as detailed here - Apple does not currently list any other supported carriers or countries for the models that are listed for China:
Apple Watch - Carriers - Apple
If in doubt, I suggest that you contact your intended carrier to ask whether any given model can be used on their network with a plan that they may offer or plan on offering for iPhone that supports Apple Watch Series 3 (GPS + Cellular).
Apple Watch Series 3 (GPS + Cellular) can be used without activating the built-in cellular feature, in which case it functions like a GPS-only model, in conjunction with an iPhone 6 or newer model of iPhone (running the latest version of iOS) and a service plan from any carrier.
The built-in cellular features cannot be used when roaming.
The warranty terms for an Apple Watch purchased in China are detailed here:
Both
The functions that are available on your Apple Watch when it is away from your iPhone depend on various factors including:
- Your Apple Watch model - Series 3 (GPS) vs Series 3 (GPS + Cellular):
- The features of your carrier service plan:
- The type of connectivity that is available to your Apple Watch at the time:
About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support
Features and requirements that apply under various circumstances are detailed here, including within the footnotes:
How to use your Apple Watch without your iPhone nearby - Apple Support