Will Samsung Pay still work on the Gear S3 if you root your Galaxy phone?
According to the official web page... LTE watches work after an initial pairing with your smartphone &amp; Samsung Pay works after initial set up with your phone and then you only get 5 payments with the watch without a phone before it needs to reconnect and re pair with your phone.

source: http://www.samsung.com/us/explore/gear-s3/?cid=ppc- 

1* 4G LTE standalone connectivity only available on Samsung Gear S3 frontier version, a companion for compatible Android smartphones, sold separately. Standalone connectivity requires initial pairing of Gear S3 frontier with compatible phone and separate qualifying wireless plan. Standalone functionality limited if paired phone is not powered on or connected to a wireless network. Network coverage not available everywhere. Consult carrier for details.

2* Make up to five payments on your Gear S3 without a Galaxy smartphone nearby after initial setup. Reload payment capability by connecting to a Galaxy smartphone via Bluetooth. Only compatible with select cards, carriers and Samsung devices; see the Samsung Pay Support page http://www.samsung.com/us/support/owners/app/samsung-pay#compatibility for compatibility information.