Hey there.
Welcome to Three Community.
If you already have the watch, you just need to start the pairing process, the watch and phone will lead you to the My3 page where you can purchase the addon for smartwatch pairing. It's currently £7 per month to add it to your plan, and is a rolling monthly add, so can be removed if you no longer need it.
For Galaxy Watches, you'll need to be using a Galaxy Phone in order for pairing to be compatible.
Pete.
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