Hi
Apple Watch (GPS + Cellular) release in Brazil
Apple advises that Apple Watch Series 3 (GPS + Cellular) will become available in other countries next year, but it has not yet specified when and where.
I suggest that you monitor the Apple Online Store for further information regarding availability in due course:
Watch - Apple (BR)
I'm afraid that, in the meantime, no-one here can tell you anything about Apple's unannounced plans. You are not talking to Apple here - this is a user-based technical support community.
Buying abroad
Apple Watch (GPS + Cellular) is not locked to any specific carrier and has a built-in eSIM that copies information from the iPhone's SIM card.
The built-in cellular features require a carrier plan in your country of use that specifically includes support for Apple Watch (including the carrier activating the device on the plan).
If buying Apple Watch Series 3 (GPS + Cellular) in another country, then you would need to buy a model that supports the relevant LTE and UMTS bands for your intended country of use and your supported carrier there.
There are three different regional versions of Apple Watch Series 3 (GPS + Cellular), each supporting different LTE and UMTS bands, as detailed here:
Apple Watch - Carriers - Apple
If in doubt, I suggest that you contact your intended carrier to ask whether any given model can be used on their network with any suitable service plan that may offer or plan on offering.
Note that the built-in cellular features cannot be used when roaming.
Also keep in mind that warranty service may be restricted to the country in which Apple Watch is originally purchased:
Apple One (1) Year Limited Warranty