Hi
Alongside Apple Watch Series 1, there are two different versions of Apple Watch Series 3:
Apple Watch Series 3 (GPS) and Series 1
Apple Watch Series 3 (GPS) and Series 1 models do not have built-in cellular capabilities.
Any data consumed by Apple Watch Series 3 (GPS) is consumed either via the paired iPhone (including Wi-Fi and cellular data) 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. An iPhone 5s or newer model of iPhone is required, running the latest version of iOS.
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.
In order to be able to use the built-in cellular feature, you would require a carrier service plan for your iPhone that specifically includes support for the Apple Watch, including the carrier first activating the device on the plan:
- Set up and use cellular on Apple Watch Series 3 (GPS + Cellular) - Apple Support
- Use Apple Watch with a cellular network—Apple Watch Series 3 (GPS + Cellular) only
Apple's list of supported carriers is linked below. Note that Straight Talk is not currently listed:
Apple Watch - Carriers - Apple
You may wish to contact Straight Talk to ask about their plans for offering service plans for iPhone that include support for the built-in cellular feature on Apple Watch Series 3 (GPS+ Cellular).
Apple Watch Series 3 (GPS + Cellular) can also be used without enabling the built-in cellular feature, in which case it functions like a GPS-only model and can be used in conjunction with a service plan for iPhone 6 or newer model of iPhone, running the latest version of iOS, from any carrier, with no need to activate the watch on the plan.
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:
- 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