Samsung Pay will work on Gear S3 watches paired with any phone running 4.4 KitKat and up
Unfortunately, this doesn't mean that Samsung Pay will be made available on all non-Samsung smartphones as a standalone service, but it's a liberating decision on part of the company nonetheless, as Gear S3 users won't be forced to have their smartwatches paired to a Samsung device in order to use the company's wireless payment platform.
Inputting card info will likely be handled by some sort of a companion app, rather than from the watch itself, although Samsung is known for striving for the highest security possible when it comes to its wireless payment service, so it remains to be seen how this will pan out.
The information has been confirmed by a Samsung Pay rep on Twitter, but details are slim at this point. More info should surface in the following weeks.
source: Samsung via AndroidCentral