Apple Watch 3 + cellular and Apple Pay

See this support document for a list of things you can do without your iPhone nearby when using your Apple Watch. How to use your Apple Watch without your iPhone nearby - Apple Support

This support document also indicates how to set up Apple Pay on your Apple Watch. Set up Apple Pay - Apple Support

Hi

Yes - once it has been set up, Apple Pay can be used on Apple Watch (any model) entirely independently of the paired iPhone, including when the iPhone is not nearby and/or not turned on.

Apple Watch also does not need Bluetooth, cellular or Wi-Fi connectivity when paying via Apple Pay in stores. It can be used for that purpose, for example, when in Airplane Mode.

More information:

Using Apple Pay in stores, within apps, and on the web - Apple Support

Set up Apple Pay - Apple Support