Do you have to buy a data plan for a series 3 watch or does it also work like the series 2 if you have your Apple phone with you?

Hi

There are two different versions of Apple Watch Series 3:

• Apple Watch - Compare - Apple
• Apple Watch Series 3 - Apple

Apple Watch Series 3 (GPS)

Apple Watch Series 3 (GPS) 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 data plan for the Apple Watch is not required.

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

An activation fee and ongoing monthly cost may apply - contact your carrier for details:

Apple Watch - Carriers - Apple

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.

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):

  Apple Watch - Compare - Apple

• The features of your carrier service plan:
  • Apple Watch - Carriers - Apple
  • Make a call with Wi-Fi Calling - Apple Support
• 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

They have two versions -- GPS and GPS + Cellular. The GPS only versions are a bit cheaper to buy if you don't want cellular. See: Buy Apple Watch Series 3 - Apple