Series 3 Watch Need App

you should be able to use all the build in apps and the apps from app store, that do not require a newer version of WatchOS then the series 3 support

the highest version it supports is watchOS 8.8.1

the latest version of watchOS is 26

so I would not recommend you purchasing a series 3 if that is what you are asking

people report it's possible to pair newer iphones with older watches

How can I connect Apple Watch Series 3 to… - Apple Community

what AI says

"Yes, you can pair an Apple Watch Series 3 with an iPhone 16, but you must set up the watch as a new device and follow the standard pairing process using the Apple Watch app on your iPhone. Ensure both devices are charged and have the latest software updates installed. 

Before you begin

• Unpair the watch: If the Apple Watch is already paired with a previous phone, you must unpair it first by going to the Apple Watch app on that phone, tapping "All Watches," then the information icon next to the watch, and selecting "Unpair Apple Watch".
• Ensure updates are current: Check that your iPhone 16 has the latest iOS version and that your Apple Watch has the most recent watchOS compatible with the Series 3. 

Step-by-step pairing guide

1. Turn on your Apple Watch: Put the watch on your wrist and press and hold the side button until the Apple logo appears.
2. Open the Apple Watch app on your iPhone: Keep your Apple Watch and iPhone close together. The "Use your iPhone to set up this Apple Watch" message should appear on your iPhone; tap Continue.
3. Start pairing: If the message doesn't appear, open the Apple Watch app, go to the "All Watches" tab, tap "Add Watch," and then "Set Up for Myself".
4. Use the camera: Position the Apple Watch in the viewfinder on your iPhone's screen until it pairs automatically.
5. Pair manually (if needed): If the camera doesn't work, tap "Pair Apple Watch Manually" on your iPhone and follow the on-screen prompts.
6. Finish the setup: After pairing, follow the instructions to set up as a new watch, create a passcode, and complete the process. Do not restore from a backup from a newer watch model, as this may cause issues. 

"

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apple watch 3 pair with iphone 16 - Google-

if chatgpt is right or not may be contested, all previous compatability between watchOS and ios have always been in the form of "watchOS 8 requires an iPhone running iOS 15" (or newer)

if it was me then I would do some more investegation and follow some guides and test out stuff before purchasning a new watch, if you are content with the features your current watch s3 offer.

ChatGPT is wrong, as it often is.